Cow-asthmatic farmers' and negative control subjects' IgG and IgE antibody responses to bovine epithelial antigen (BEA) and urinary antigen (BUA) were studied by enzyme-linked immunosorbent assay (ELISA) and Western blotting. The anti-BEA IgE responses of 10 highly reactive sera were also studied by crossed radioimmunoelectrophoresis (CRIE). The relative amount of allergens common to both BEA and BUA was measured by IgE ELISA inhibition and found to be 3%. In immunoblotting the IgG reactivity of the asthmatic farmers to BEA and BUA declined along their anti-BEA IgE ELISA titres. Control subjects had IgG antibodies mainly to high molecular weight components (50-70 kD) but lacked detectable IgE responses. The IgE reactivity of the asthmatic farmers was directed to only a few components. A total of two main allergens were found in cow dander (20 and 22 kD) and one in cow urine (20 kD). The 20 kD component was shown to be the most important allergen in cow antigen extracts. In CRIE, seven reactive arcs were detected. Arcs 1, 2 and 5 were detected by all 10 sera and are 3 by six and arc 7 by seven sera.