Since finding that all subjects with contact allergy to cocamidopropylbetaine give positive reactions to 3-dimethylaminopropylamine (DMPA), we wished to verify whether sensitization to other industrially-used tensioactives might also be due to content of DMPA as an impurity. We also investigated the possible "carrier action" that tensioactives might exert on minimal quantities of DMPA. Finally, we analyzed the relationship between the structure of DMPA and other chemically-correlated molecules and their sensitizing potential, with particular reference to the structure of alkylamidopropylbetaines. For this purpose, in 34 patients with contact allergy to DMPA, we tested: (i) DMPA in concentrations below the threshold limit in water and in different tensioactives; (ii) substances that employ DMPA as a reagent in their synthesis; (iii) substances similar to DMPA as regards chemically reactive groups. The study showed that: (i) DMPA remains as a quantitatively detectable impurity in all tensioactives employing it in their synthesis; (ii) some common anionic (SLES) and non-ionic (polysorbate 20) tensioactives enhance the risk of sensitization from very low doses of DMPA, presumably due to a "carrier effect;" (iii) the sensitizing chemical structures in DMPA and related molecules are the primary amine and the tertiary (dimethyl-substituted) amine groups, when separated by either 2 or 3 carbon atoms; (iv) no sensitizing action can be attributed to the functional groups present in alkylamidopropylbetaine molecules.