Abstract: Male Clean SD rats were divided into two groups, and raised in conventional facility (open air,group 1 ,n=54) or in a room utilizing laminar air flow with filtration (clean air, group 2,n=50). To induce immunosuppression the rats were given cortisone acetate (25mg×2/week，sc)for 12 weeks, then the lung impression smears and bronchoalveolar lavage fluid sediment smears stained with Giemsa stain and lung sections stained with Gro-cott-Gomori silver stain were prepared to find Pneiunocystis carinii (PC) organism. Bacteria and fungi were also checked. The results showed that the incidence of induced Pnenmocystis carinii pneumonia (PCP) in group 2 (28%, 14/50) was higher then that in group 1 (0% ,0/54,P＜0.001),and that the incidence of bacterial pneumonia in group 2(62%) was lower than that in group 1 (89% ,P＜0.01). The results suggested that the air quality of breeding room could affect the incidence of PCP induced by immunosuppressive agents in rat.