17 letters

bacteriophagology bacteriorhodopsin balanoblennorrhea balneotherapeutic basiarachnoiditis basiparachromatin bathyhypoesthesia bathyorographical bathythermography bathythermosphere batrachomyomachia

18 letters

bacterioagglutinin bacteriofluorescin bacterioprecipitin bacterioscopically bacteriostatically ballistocardiogram balneotherapeutics barytostrontianite basidiobolomycosis basoerythrocytosis basometachromophil bathyconductograph bathyhyperesthesia bathythermographic

19 letters

bacteriochlorophyll bacteriopheophorbin bacteriotherapeutic ballistocardiograph barothermohygrogram basicytoparaplastin

20 letters

ballistocardiography Bandenkampfabzeichen barothermohygrograph batrachoherpetomachy batrochoherpetomachy

21 letters

ballistocardiiography ballistocardiographic batrachoherpetomachia

22 letters

bathysiderodromophobia

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