Skin tests and in vitro histamine-release reactions were used to evaluate 130 patients observed in an employee allergy clinic at a biomedical research facility. The allergens used included extracts from pollens (ragweed, grasses, trees, weeds), molds, mixed feathers, house dust, cat, dog, mouse, rat, rabbit, guinea pig, and hamster. Of all patients, 66% complained of allergic symptoms on laboratory animal exposure, although only 52% worked directly with animals. Among patients with symptoms, 91% were positive by skin test to at least one laboratory animal, and 46% had asthma. The median length of exposure to laboratory animals before onset of symptoms was 2.8 yr with 60% of the patients developing their symptoms within 3 yr. Among patients who had allergic symptoms before exposure to laboratory animals, 79% were skin test positive to laboratory animals when they were evaluated in this study. There was a close association found between the skin test and histamine-release results with the laboratory animal allergens: 91% of the 4+ skin reactors had leukocytes positive for histamine release versus 5% of the leukocyte donors with less than 1+ skin reactions. A close relationship in positive reactions to different laboratory animal allergens was also found. For example, individuals positive to mouse were positive also to rat (95%), rabbit (79%), guinea pig (83%), and hamster (88%). Patients who reacted to laboratory animals also reacted to some extent to house dust and cat and dog allergens, and about one half of the animal-allergic individuals reacted to pollens. Although nonpollen-allergic individuals can develop sensitivity to laboratory animals, the group at higher risk are allergic individuals, especially those sensitive to house dust, cats, or dogs.