From 1981 to 1987, 26 outbreaks of asthma have occurred in the city of Barcelona, Spain, affecting a total of 687 subjects and causing 1155 emergency room admissions. Assays of urban aerosols collected with high-volume samplers between October 1986 and May 1989 have indicated that soybean dust originating from harbor activities (and not traffic or industrial pollution) is the causal agent for these epidemics. Soybean particulates in the filters have been characterized from the composition of the alcohol fraction, namely, campesterol, stigmasterol, beta-sitosterol, n-triacontan-1-ol, and n-dotriacontan-1-ol, constituting a series of aerosol components correlated with the epidemiologic data. This result has also been confirmed by immunochemical assay of specific soybean allergens. The concentration of these sterols in the air corresponds to a 24-hour average level of soybean dust in the order of 25 micrograms/m3 on epidemic days. These results suggest the advisability of monitoring soybean dust in air particulates of populated areas surrounding soybean storage or processing plants. The techniques presented here afford a simple way for the recognition of soybean dust in aerosols containing high concentrations of organic pollutants of traffic or industrial origin.