Symptomatic and immunologic responses to allergens from laboratory mice were studied in a research institute. Subjects who had been exposed to mice and 50 unexposed subjects were studied by questionnaire and by prick tests with seven prevalent aeroallergens and allergens from mouse urine and pelts. Of the 121 exposed subjects, 39 (32.2%) had respiratory, ocular, or cutaneous symptoms after exposure to mice; occurrence of these symptoms correlated with positive skin tests to purified mouse urinary proteins (MUP) and pelt allergens from CBA/H mice. Serum levels of IgG antibodies correlated with the frequency of mouse exposure. In subjects with seasonal allergic rhinitis, nasal symptoms from exposure to mice, positive prick tests to MUP, and IgE antibodies to MUP were significantly more prevalent. The possibility of genetic influences on susceptibility to mouse allergy were also suggested by a negative association between the incidence of HLA-DRW6 and positive prick-test responses to urinary proteins from C57BL and BALB/c mice among the 54 subjects who were exposed to mice and tested for DR locus antigens (p = 0.05). However, no significant differences in any of the loci studied could be shown in subjects with and without nasal symptoms from exposure to mice.