To investigate whether successful host defense against Pneumocystis carinii is dependent on induction of inducible nitric oxide synthase (iNOS) in alveolar macrophages, immunocompetent mice, mice depleted of CD4 lymphocytes with anti-CD4 antibody, and mice with severe combined immunodeficiency (scid) were inoculated intratracheally with P. carinii. Three weeks later, immunocompetent mice had cleared the organisms completely, while CD4 cell-depleted and scid mice were severely infected (scores, 3.6 ± 0.2 and 2.8 ± 0.2, respectively). Inflammation scores were significantly higher in CD4 cell-depleted mice (3.4 ± 0.2) than in scid mice (0.6 ± 0.2). Minimal iNOS mRNA was detectable in lung tissue from immunocompetent mice; iNOS mRNA was comparable in scid mice and mice inoculated with PBS but was 6-fold higher in CD4 cell-depleted mice. Immunohistochemistry localized iNOS protein to alveolar macrophages in CD4 cell-depleted mice. Thus, iNOS is an unlikely participant in host defense against P. carinii, because enzyme expression does not correlate with either clearance or severity of infection.