Mammoth Uncensored Word List:
15 words ending with ls, 16 letters long

chloramphenicols cholecalciferols counterproposals dessertspoonfuls duopentagesimals hexylresorcinols microsporophylls musculoskeletals nonintellectuals nonprofessionals organomercurials pentoctogesimals phosphoglycerols septemvigesimals subprofessionals