Twenty patients allergic to cow's milk proteins and with high levels of specific IgE directed against bovine whole casein were selected to evaluate reactivity of their IgE antibodies with human beta-casein. Highly purified human and bovine beta-caseins were prepared by selective precipitations and FPLC separation. Their identity and purity were assessed by HPLC, analysis of amino acid composition, sequencing of the five N-terminal amino acid residues and immunochemical tests. Direct and indirect ELISAs were performed using human and bovine beta-casein coated into microtiter plates and monoclonal anti-human IgE antibody AChE labelled for revelation. Seven sera contained specific IgE directed against human beta-casein. Inhibition studies using native human and bovine beta-caseins as well as bovine beta-casein-derived peptides demonstrated that, depending on the sera, one or several common epitopes located in different parts of the molecule were shared by the two homologous proteins.