The recruitment of specific subsets of CD4⁺ T lymphocytes to the lungs in response to Pneumocystis carinii was investigated. For mice inoculated with P. carinii, an ELISPOT assay was used to calculate the numbers of lymph node and lung tissue CD4⁺ cells that secreted interferon (IFN)-γ (Th1 cytokine) and interleukin (IL)-4 (Th2 cytokine) after concanavalin A stimulation. An ELISA was used to assay culture supernatants for cytokine concentrations. Precursor frequency of both IFN-γ- and IL-4-secreting cells was increased in lymph nodes at 1 week, whereas increases in Thl and Th2 cells in lung tissue were delayed 3 weeks before declining. The frequency of IL-4-secreting cells always was greater than the frequency of IFN-γ secreting cells. These results demonstrate an early T lymphocyte response in draining lymph nodes, followed by later recruitment of Thl and Th2 lymphocytes into lung tissue. The overall CD4⁺ T cell response to P. carinii involves both Thl and Th2 subsets, but the response is Th2 dominant in both lymph node and lung tissue.