15 letters

photoconvulsive incomprehensive ∙ uncomprehensive ∙ nonapprehensive misapprehensive ∙ antihypotensive radioresponsive hyperresponsive ∙ interresponsive nonintroversive hyperaggressive ∙ semiprogressive antiprogressive ∙ autoprogressive unretrogressive superexpressive nonadjudicative nonmodificative exemplificative ∙ insignificative ∙ unsignificative cosignificative ∙ personificative incommunicative ∙ uncommunicative ∙ excommunicative ∙ prognosticative ∙ nonintoxicative Executrixdative electronegative ∙ noncongregative uninvestigative uninterrogative nondepreciative nonappreciative misappreciative ∙ nondenunciative superinitiative nonpropitiative differentiative nonassimilative disassimilative ∙ uninterpolative uncontemplative semilegislative semispeculative antispeculative ∙ overspeculative subarticulative nonarticulative ungesticulative anticoagulative overstimulative nonaccumulative cirrocumulative nonmanipulative phosphorylative ∙ nondesquamative overaffirmative nonconfirmative unsubordinative overimaginative ∙ decontaminative ∙ uncontaminative unrecriminative indeterminative nonilluminative procrastinative ∙ nonopinionative nonanticipative unparticipative nonemancipative unreverberative nonexaggerative nonaccelerative unagglomerative nondegenerative nonregenerative retrogenerative intergenerative nonremunerative nonrecuperative nonvituperative uncommiserative nonalliterative uncollaborative uncorroborative undeteriorative uncommemorative unconcentrative undemonstrative ∙ unremonstrative nonillustrative transfigurative postsuppurative subcompensative noncompensative neurovegetative unresuscitative nondebilitative nonexploitative ∙ unprecipitative inauthoritative unauthoritative ∙ nonquantitative ∙ nonconsultative complementative nonaugmentative complimentative experimentative ∙ nonfermentative unargumentative instrumentative unfrequentative unmanifestative anticommutative interderivative nonpreservative nonconservative ∙ Neoconservative ∙ cholinoreactive photorefractive ∙ superattractive schizoaffective Intersubjective ∙ transsubjective stereoselective cardioselective retroreflective ∙ nonrecollective unintrospective radioprotective ∙ chemoprotective neuroprotective uncontradictive nonconstrictive photoconductive ∙ superconductive ∙ prereproductive nonreproductive ∙ underproductive subintroductive semidestructive autodestructive ∙ overdestructive overinstructive nonconstructive ∙ misconstructive interinhibitive pseudoprimitive thermosensitive pressosensitive electropositive ∙ anticompetitive ∙ overcompetitive seminvariantive neuroaugmentive pressoreceptive antinociceptive saccharoceptive photoperceptive noninterceptive intussusceptive ∙ locodescriptive overdescriptive nonprescriptive circumscriptive ∙ proconscriptive nonproscriptive overpresumptive noninterruptive pseudoassertive fibrocongestive precontributive noncontributive nondistributive nonsubstitutive

16 letters

turbinatoconcave counteroffensive precomprehensive noncomprehensive overapprehensive antihypertensive pseudoaggressive nontransgressive myelosuppressive contraindicative presignificative consignificative nonsignificative unmultiplicative noncommunicative overappreciative nonappropriative noninterpolative noncontemplative hypersuperlative hyperspeculative automanipulative untransformative noncontaminative indiscriminative undiscriminative predeterminative nondeterminative nonexterminative hemagglutinative nonagglutinative isoagglutinative unconglutinative nonagglomerative noncommiserative noncollaborative noncorroborative noncommemorative nonincorporative counternarrative interpenetrative nonconcentrative unadministrative predemonstrative nondemonstrative overillustrative uninterpretative nonresuscitative nonprecipitative nonauthoritative histioirritative semiquantitative antifermentative homofermentative nonargumentative unrepresentative quoddamodotative quadriderivative semiconservative anticonservative overconservative antiputrefactive noncounteractive resinoextractive uncircumspective nonintrospective cardioprotective counterinvective vasoconstrictive electrostrictive magnetostrictive postreproductive cyclodestructive interdestructive parthenogenitive philoprogenitive superinquisitive Radioinsensitive meteorosensitive electrosensitive counterintuitive mechanoreceptive electroreceptive nontranscriptive anticonscriptive

17 letters

electroconvulsive electrodispersive immunosuppressive electropercussive nonmultiplicative pseudoapplicative semicommunicative overcommunicative unprognosticative photphotonegative photodissociative subtersuperlative pseudolegislative overgesticulative nondiscriminative antiagglutinative haemagglutinative anticonfederative antiproliferative Neurodegenerative superregenerative subadministrative preadministrative maladministrative nonadministrative overdemonstrative preinterpretative noninterpretative overargumentative nonrepresentative misrepresentative ultraconservative superconservative hyperconservative counterattractive pseudoperspective chondroprotective contradistinctive counterproductive pseudocompetitive Froschperspektive

18 letters

pseudoapprehensive intercommunicative nonprognosticative transubstantiative myeloproliferative heterofermentative archrepresentative omnirepresentative overrepresentative unltraconservative pseudoconservative noncircumscriptive

19 letters

lymphoproliferative immunoproliferative erythrodegenerative thermoelectromotive

20 letters

polyphiloprogenitive

21 letters

anerythroregenerative

22 letters

PersonalRepresentative

Words with a ∙ are valid Scrabble words (SOWPODS word list)