Abstract: Background: Mouse urinary allergens are an important cause of occupational asthma in animal facilities. Domestic exposure to mouse allergens is a risk factor for asthma among inner-city residents. Objective: We sought to develop a sensitive and specific assay for assessing environmental mouse allergen exposure. Methods: An ELISA for recombinant (r)Mus m 1 was developed by using rabbit polyclonal antibodies to rMus m 1 that were affinity purified against the natural allergen. Assay specificity was established by means of immunoblotting and ELISA. Mus m 1 levels in mouse, other mammalian allergenic products, and house dust samples from inner-city homes were compared. Results: Polyclonal antibodies to Mus m 1 showed a single 20-kd band on immunoblots against rMus m 1 and male mouse urine. Parallel dose-response curves were obtained by using mouse urine extract and natural Mus m 1 or rMus m 1. Mus m 1 was detected in mouse allergenic products (0.10-10.0 mg/mL) and in gerbil allergenic products (0.1 mg/mL) but was less than the limit of detection in epithelial extracts from 10 other animal species. Environmental measurements showed an excellent correlation between Mus m 1 levels in house dust extracts from inner-city asthma studies by using 2 different Mus m 1 standards (n = 22; r = 0.99; P < .001). Conclusions: A highly sensitive ELISA has been developed with rMus m 1. This assay is suitable for monitoring domestic and environmental exposure to mouse urinary allergens.