179 people working with small laboratory animals were studied by questionnaire, lung-function tests, skin tests with common allergen extracts and extracts of the serum and urine of five animal species, and serology. 49 people had symptoms related to animal contact, most commonly rhinitis but also asthma and skin rashes. Skin reactivity to animal extracts was related to symptoms, particularly asthma, but many people with rhinitis only had negative skin tests. Positive skin tests without symptoms were rare. 3 people had additional late skin reactions. Atopic individuals were no more likely than non-atopics to have animal-related symptoms but were more likely to have asthma. Gel diffusion revealed no precipitating antibodies, and no specific IgG was detected with the ELISA technique. Lung-function measurements were normal. Sensitisation to laboratory animals may involve heavy antigen exposure, so that relatively unreactive individuals can be affected. Exclusion of atopic individuals from work with laboratory animals will not eliminate the problem.