Mammoth Uncensored Word List:
18 words ending with ts, 19 letters long

anarchosyndicalists antivivisectionists dendrochronologists disacknowledgements disenfranchisements ethnomethodologists gastroenterologists gedankenexperiments hydrometeorologists immunohematologists micrometeorologists noninterventionists paleoclimatologists preacknowledgements psychoimmunologists psychophysiologists rhinolaryngologists sclerochronologists