The major allergens from the mouse, including mouse serum albumin (MSA) and mouse urinary protein complex, are present in various concentrations in urine, serum, and pelts of mice. Subjects clinically sensitive to laboratory animals were screened by the RAST for specific IgE antibodies to mouse urine (MU), MSA, and mouse pelt extract (MPE). Twenty-four hour air samples from both a mouse-care room (2000 mice) and an immunology laboratory (five to 100 mice) were collected with a high-volume air sampler that retained particles greater than 0.3 micron with 95% efficiency. Eluates from the filter sheets were dialyzed, lyophilized, reconstituted in water, and used as fluid-phase inhibitors in the MU RAST and MPE RAST. MPE allergenic activity could be quantitated in filter eluates from both locations, but MU allergenic activity could be quantitated only in the mouse-care room. Airborne allergen content ranged from 1.8 to 825 ng/m3 and varied with both the number of mice and the degree of work activity in the rooms. The sampling and assay techniques described can be used to accurately quantitate amorphous airborne allergens.