Although a clear relationship between alphabeta T-cell receptor-positive (alphabeta-TCR(+)) CD4(+) T cells and susceptibility to Pneumocystis carinii infection exists, the role of other T-cell subsets is less clearly defined. Previous studies have shown that gammadelta-TCR(+) T cells infiltrate into the lung during P. carinii pneumonia. Therefore, the present study examined the role of gammadelta-TCR(+) T cells in host defense against P. carinii pneumonia. C57BL/6 (control) and B6.129P2-Tcrd(tm1Mom) (gammadelta-TCR(+) T-cell-deficient) mice were inoculated intratracheally with P. carinii. At specific time points, mice were sacrificed and analyzed for P. carinii burden, T-cell subsets, and cytokine levels in lung tissue. Analysis of P. carinii burden showed a more rapid and complete resolution of infection in gammadelta-TCR(+) T-cell-deficient mice than in C57BL/6 controls. This augmented resolution was associated with elevated gamma interferon (IFN-gamma) levels in bronchoalveolar lavage fluid predominantly produced by CD8(+) T cells, as well as an increased recruitment of CD8(+) T cells in general. In separate experiments, neutralization of IFN-gamma or depletion of CD8(+) T cells early during infection abolished the augmented resolution previously observed in gammadelta-TCR(+) T-cell-deficient mice. These results show that the presence of gammadelta-TCR(+) T cells modulates host susceptibility to P. carinii pneumonia through interactions with pulmonary CD8(+) T cells and tissue production of IFN-gamma.