A mouse model for Pneumocystis carinii has now been established in several strains of mice: C3Heb/FeJ, C3HeN, Balb/c, DBA/2N and athymic. In lieu of using invasive methods for initiating P. carinii infections, mice infected with P. carinii (seed mice) transmitted the disease to mice without latent infection via short term co-habitation. Acute infections in recipient mice developed approximately 5-6 wk after C3Heb/FeJ seeds were removed, while control unseeded litter-mates remained uninfected. This approach allows investigators to consistently transmit P. carinii to mice and to select the strain of mouse desired for use in a particular study.