Three types of air-sampling apparatus (the Air-Sentinel, Burkard Personal Sampler and Casella Personal Sampler) were compared for their use in sampling the air in situations where individuals are exposed to laboratory animals, and a cascade impactor was used to determine the sizes of particles carrying the allergens under investigation. An ELISA, using monospecific antisera raised to selected major allergens of the mouse, rat and rabbit, i.e. mouse Ag 1 (mouse urinary prealbumin), rat Ag 4 (rat urinary prealbumin) and rabbit Ag R1, was developed to assay the extracts obtained from the samplers. This ELISA system was able to detect greater than 5 ng/m3 of allergen in sampled air. The rat and mouse allergens were shown to be carried mainly on particles of 6-18 micron, whereas the rabbit allergen was also found on particles of 2 micron and smaller. Allergen levels correlated well with the number of animals present in the room and the degree of activity during sampling. A protective filtered-air hood, when worn during surgical operation procedures, was shown to reduce effectively the level of rat allergen breathed by an individual.