The pattern of incidence of allergy to laboratory animals (ALA) has been studied prospectively in 383 individuals occupationally exposed to rodents and to rabbits. The incidence of the disease after one year of exposure to animals fell from 37% in 1980 and 1981 to 20% in 1982, 10% in 1983, and 12% in 1984. A similar reduction was noted after two and three years of exposure in the 1982 and 1983 cohorts. The reduction in incidence coincided with the introduction of a site order and code of practice for working with animals and an education programme designed to focus awareness on the problem. Although rats were believed to be the major cause of the disease, objective measurements of IgE antibody against rat urine allergen were positive in only half the symptomatic individuals. Antibodies to different animal derived allergens were, however, found in several other personnel. Symptoms were generally of mild or moderate intensity and affected mainly the nose, eyes, and skin; chest symptoms were found in only 1.6% of the exposed population. A study of the influence of atopy on the development of ALA showed that after one year of exposure to animals a significantly greater proportion of atopic individuals became symptomatic (19-43% compared with 3-6%). After two and three years of exposure, however, this discrepancy was not maintained, with more non-atopic individuals developing the disease.