The allergic potential of α-caseins from bovine, ovine, and goat's milk sharing more than 85% identical amino acids was compared. Caseins were purified by anion-exchange chromatography and used for a specific IgE and IgG ELISA with diluted human sera. Sera were from 17 children with immediate-type allergy to cow's milk, from 59 children with atopy but without food allergy, and from 27 healthy children without atopic disease. The sera of cow's milk-allergic children showed a significantly higher IgE and IgG binding to α-caseins from all three species than the sera of the other groups. All groups showed an increased antibody binding to bovine a-casein compared to the sheep and goat proteins, but the differences were significant only in the groups of atopic children and of healthy controls. Furthermore, inhibition of the IgE binding to bovine α-casein with α-casein from cow, goat, and sheep revealed that the a-caseins from these species are highly cross-reactive, on the basis of the small differences in their primary structure. In conclusion, the milk of goat and sheep harbor an allergic potential and is not suitable for the nutrition of milk-allergic patients.