To determine the effect of humidity on the levels of the mouse allergen Mus m 1, an experimental animal room was constructed to control environmental variables. The sex, strain, age, and number of mice was constant in the room, so that the average daily production of Mus m 1 would not vary greatly. Six different levels of relative humidity from 15% to 65% were maintained for a minimum of a week each. Daily collections of airborne particulates were eluted from filters and Mus m 1 content measured by immunological assay. Increasing relative humidity caused a decrease in Mus m 1 levels from a high of 3 ng/m3 at 15% humidity to a low of 0.5 ng/m3 at 65% humidity. Thus, reduction of airborne allergen levels can be achieved by careful attention to humidity control, especially during the winter heating season when humidity levels may be low. This experimental room can be used to measure the effect of other variables such as ventilation rate, caging, bedding, and work practices on the levels of mouse allergen in an animal facility.