In the past year, 1200 consecutive eczematous patients were tested with cocamidopropylbetaine 1% aq. Contact allergy was evinced in 46 subjects (3.8%), while irritant reactions (slight erythema only) were observed in 15 cases (1.25%). 30 out of 46 patients with allergic reactions were subsequently tested with the substances used in the synthesis of cocamidopropylbetaine, together with a sample of cocamidopropylbetaine declared by the supplier to possess a greater purity. In all 30 subjects, positive reactions were obtained to 3-dimethylaminopropylamine (DMPA) 1% aq., while the cocamidopropylbetaine defined of purer grade, at 0.5% and 1% aq., gave positive reactions in 10% and 53% of cases, respectively. These results suggest that the DMPA present at various levels as an impurity in the commercial product is responsible for cocamidopropylbetaine allergy. Owing to the inconsistency of positive reactions to cocamidopropylbetaine of variable purity, and to the consistency of positive reactions to DMPA, it seems likely that these reactions may also be connected with the presence in the product, defined of purer grade, of unknown amounts of DMPA as impurity. Structural similarities between the 2 molecules cannot be considered in this case, because the DMPA structure is radically changed in its transformation to the betaine structure. Further experiments with other molecules related to the above structures are in hand.