A survey of North, Central and South American composites, and of certain polyacetylenic compounds occurring in them, confirmed that the UV-mediated antibiotic activity against Candida albicans can be ascribed to the presence of particular polyacetylenes and their thiophene derivatives. Leaves, stems, roots and achenes were assayed separately. An attempt was made to relate the phototoxic activity of specific compounds to their chemical structures. Most composites tested were not phototoxic against Candida, but many were antibiotic. The antibiotic activity is apparently caused by polyacetylenes. Of 65 sesquiterpene lactones assayed, only a few were antibiotic, and only 1, glaucolide G, was phototoxic.