We studied the binding activities of IgE and IgG antibodies in patients with allergy to cow milk proteins, against different alpha-casein preparations: alpha-casein treated with urea, hydrochloric acid, sodium hydroxide, or sodium dodecyl sulfate (SDS); or heat-denatured alpha-casein. The binding activities of IgE and IgG antibodies to these denatured alpha-casein preparations were compared with those to native alpha-casein. The binding activities of IgE and IgG antibodies to these denatured alpha-casein preparations were similar to those to native alpha-casein although the binding activities of IgG antibodies to these denatured alpha-casein preparations were relatively heterogeneous compared with those of IgE antibodies. Since modifications of alpha-casein did not alter the ability of alpha-casein to react with these antibodies, IgE and IgG antibodies to alpha-casein in sera from patients with allergy preferentially bind to the antigenic determinants associated with primary protein structures.