The factors influencing airborne allergen exposures in a laboratory animal house were examined. Breathing zone samples were collected on 21 animal technicians in an animal house over a 4 day period while they performed their usual work tasks. The samplers were changed each time a different task was performed and were analyzed for rat urinary aeroallergen (RUA). During the sampling periods, all rats in each of the 19 rooms were counted, the size of each room, the inlet air ventilation and the outlet air ventilation were measured, and the type of bedding used was recorded. From these variables, the number of female and male rats, the number of rats per room volume (stock density), number of rats per unit ventilation (stock density/inlet), number of rats per unit outlet ventilation (stock density/outlet), and number of air changes in the inlet and outlet ventilation systems (ACout) were calculated. The data were analyzed by logistic regression techniques to assess the influence of the various measured parameters on air RUA concentrations. The overall geometric mean RUA exposures during rat handling, cage cleaning, and indirect contact with rats were 68.0, 53.5, and 21.3 micrograms/cubic meter, respectively. The highest RUA exposures occurred on day two of the study, but reasons why RUA exposures were higher on day two than on the other days were not known. Specific job task, data obtained on day two of the study, stock density, and ACout were significantly associated with the logarithm of the RUA (lnRUA) exposures and explained 24% of the variance in the lnRUA exposure data. A model that incorporated data obtained while handling rats and cleaning cages, data obtained on day two of the study, stock density, and ACout explained 21% of the variance in the lnRUA exposure data. This model was used to predict RUA concentrations on other days of the study and indicated that increasing stock density, handling rats, and decreasing ACout were significantly associated with increasing RUA exposures. The authors conclude that the best RUA exposure assessments will be obtained by measuring exposures according to specific task performed and room.