Asthma epidemics in Barcelona, Spain, have been attributed to dust generated by the unloading of soybeans in the harbor. Sera of four different groups of 10 subjects in each group were studied: (1) patients attending an emergency room in Barcelona for an asthma attack on epidemic days, group A, (2) patients attending an emergency room for an attack on nonepidemic days, group B, (3) patients with asthma from other cities, group C, and (4) patients without asthma from Barcelona matched by age and sex with group A, group D. All subjects in group A had IgE to allergens in extracts of various soybean samples. In contrast, only one of the 10 subjects in each of groups B and C and none of those subjects in group D had IgE to uncleaned bean and hull extracts. Radioimmunoassay demonstrated that in sera from patients with asthma during an asthma outbreak reacted primarily to soybean hull and dust extracts. Sodium dodecyl sulfate-polyacrylamide and gel electrophoresis thin-layer isoelectrofocusing demonstrated protein bands of 97.4 to less than 14.4 kd and isoelectric point between 6 and 3.5. By Western blot and thin layer isoelectrofocusing/blotted radioimmunoisoelectrofocusing, IgE of patients with asthma during an asthma outbreak reacted weakly to two protein bands of molecular weight ranging from 42 to 21 kd, strongly to glycoprotein bands with molecular weight less than 14.4 kd, and isoelectric point less than 6, which appeared to be the major allergens.