The great number of topical preparation containing neomycin and their frequent use for a long period induce a lot of contact dermatitis. It is interesting to detect the importance of cross sensitivity between aminosides among the patients whose dermatitis was caused by neomycin. 15 neomycin eczematous patients were investigated. The research about cross-sensitivity concerned 6 antibiotics: streptomycin, framycetin, ribostamycin, gentamycin, kanamycin, tobramycin. Patch-tests (20 p. 100 in petrolatum) were used in order to show this cross-sensitivity. The results of these tests are nearly the same as in the other studies, as for 5 of the antibiotics; but it is the first time, to our knowledge, that a cross-sensitivity between ribostamycin and neomycin is observed. The cross-sensitivity between these two aminosides can be explained by their chemical structure. There are four rings in neomycin: the main components are neamin and neobiosamin. Only three rings are found in ribostamycin: ribostamycin is constitued of neomycin lacking neosamin of neobiosamin. Thus the structure of the 3 rings of ribostamycin is identical with the structure of 3 out of 4 rings of neomycin. The study about cross-sensitivity between aminosides suggests two conclusions: 1. The use of local antibiotics must be prescribed only when is it really necessary. 2. It seems wise to avoid all systemic aminosid antibiotics for patients who are sensitized by neomycin ointment.