The allergenicity of cow's milk whey proteins, purified by high performance liquid chromatography (HPLC), was examined by the radio‐allergosorbent test (RAST) against the sera of children immediately hypersensitive to milk. β‐lactoglobulin and α‐lactalbumin bound specific IgE in the sera of 63% and 75% of these patients respectively. These allergens were tested for cross reactivity with each other by RAST inhibition. Both inhibited the binding of IgE, in the sera of allergic patients, to the other protein. Two possible determinant peptides, one from β‐lactoglobulin and one from α‐lactalbumin, were selected by computer prediction of antigenic sites and synthesized by the fluorenylmethoxycarbonyl (FMOC)‐polyamide method. The peptides were adsorbed to nitrocellulose discs and used in further RAST studies with sera from the allergic children. Both peptides bound specific IgE in the RAST assay.