Sera from patients with cow milk protein allergy (n = 6) and cow dander allergy (n = 5) were analyzed for reactivity of IgE antibodies specific for allergens derived from milk and cow dander. The cow milk- and cow dander-allergic patients exhibited elevated specific IgE levels to milk and to cow dander, respectively, as determined by RAST. IgE immunoblot analysis revealed that cow milk-allergic patients exhibited IgE binding to the milk allergens casein, beta-lactoglobulin, alpha-lactalbumin and bovine serum albumin. Four of six cow milk-reactive patients also exhibited IgE binding to cow dander proteins with molecular weights of 20, 22, 36, 50 and > 200 kD. IgE from the sera of cow dander-allergic patients reacted with the major allergens of cow dander (20 and 22 kD) and with other proteins with molecular weights of 24, 36, 42, 50 and 70 kD. Only one out of five of these sera displayed IgE reactivity with cow milk proteins with molecular weights of 69, 92 and > 200 kD. Inhibition studies revealed the cross-reactive nature of the IgE antibodies. Preincubation of cow milk-positive sera with cow milk and subsequent immunoblotting led to complete blocking of IgE binding to cow dander proteins and to cow milk proteins. Preincubation of cow milk-positive sera with cow dander extract led to blocking of IgE binding only to defined cow milk proteins: these could be identified in 2 cases as casein and in 1 case as beta-lactoglobulin.(ABSTRACT TRUNCATED AT 250 WORDS)