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Objective: To explore the susceptibility of different strains of mice to P. carinii infection and the host immune response to the organism., Methods: C57BL/6 and BALB/c strains of mice (15 each) were exposed to SCID mice infected with P. carinii by co-housing. Observations were made on the number of parasites in the lungs, level of CD4+ T cell and CD8+ T cell in BALF, and serum IgG at 4, 5 and 6 wk after contagion., Results: The number of P. carinii grew in the lungs of BALB/c mice was found much greater than those in C57BL/6 mice. A few number of P. carinii cysts were detected in the lungs of both strains of mice by 4 wk after co-housing, the number of cysts increased at 5 wk in the lungs of BALB/c mice but not in that of C57BL/6 mice. P. carinii-specific IgG in the sera and high level of CD62low CD4- and CD8-positive T cells in the lungs were found at 5 wk. The parasites were cleared from the lungs at 6 wk in all infected mice, shortly after acquired immune response was initiated., Conclusion: BALB/c mice are more susceptible to P. carinii than the C57BL/6 mice by natural transmission, and all the immunocompetent mice can clear the infection of P. carinii by cellular and humoral immune responses.