Cross-sensitivity existed between neomycin and paro-momycin (Humatin) in 51 of 52 patients hypersensitive to neomycin, the probable primary sensitizer being framycetin ointment in one of these cases. None of the patients had been given kanamycin or paromomycin. Of 29 cases in which patch tests were performed with framycetin and kanamycin, framycetin gave positive reactions in all but one and kanamycin in 17 (59%). The reactions appeared late and were often follicular. All four antibodies have a common deoxystrept-amine moiety (l,3-diamino-4,5,6-hydroxycyclohexane) which may explain the cross-sensitivity between them.