Despite great homology with the equivalent hum an protein, bovine a-lactalbum in (Ba-La),a major com ponent of whey, has been identified as a major milk allergen. The aim of this study was to investigate the relationship between structure and IgE binding capacity in a-Las : the importance of three-dimensional structure using native vs disulfide bridge-reduced B a-La; and the incidence of amino acid sequence divergence on specific IgE cross-reactivity to human vs bovine a-La. Purified native, reduced and S-carboxymethylated Ba-La and human a-La (Ha-LA) were prepared. Specific IgE of 20 sera from patients with clinically recognized cow’s milk protein allergy and positive RAST‡ tests to B a-La were measured in original direct and competitive ELISA inhibition tests. All sera containin specific anti-native B a-La IgE also reacted with denatured protein, but the IgE levels were generally lower, showing that three-dimensional structure is an important feature in B a-La allergeni city but that sequential epitopes are also exposed after protein denaturation. Despite lower IgE levels, allsera also gave significant IgE responses to H a-La. Competitive ELISA inhibition confirmed results obtained by direct ELISA. The demonstrated IgE cross-reactivity between B a-LA and H a-La could be related to the high degree of sequence homology between the two proteins but did not prove to have a clinical significance. However, it is of great interest for a study of the relationship between structure, IgE binding capacity and allergeni city in a-Las.