Pulmonary infiltrating lymphocytes (PIL) isolated directly from human lung were examined for their surface immune phenotype by monoclonal antibody staining and cytofluorimetry. In order to purify PIL, resected lungs were enzymatically digested with collagenase and DNase and subjected to density centrifugation and nylon-wool column separation. In some cases, CD4⁺ lymphocytes were further purified with αCD8 and complement. The majority of pulmonary lymphocytes were CD2⁺ (87 ± 1%) and CD3⁺ (73 ± 4%). Virtually all of the CD3⁺ PIL were Tiαβ⁺. Greater than 90% of both CD4⁺ or CD8⁺ PIL were CD45RO⁺ and CD45RA⁻, consistent with prior antigen sensitization in vivo. A subset of CD4⁺ PIL (34 ± 4%) expressed Leu8, the human congener of the murine MEL-14 lymphocyte homing receptor, whereas most homologous CD4⁺ peripheral blood lymphocytes were Leu8⁺ (75 ± 8; P < 0.01). HLA-DR surface antigens were expressed by 45 ± 5% of CD4⁺ PIL versus 9 ± 1% of CD4⁺ peripheral blood lymphocytes (P < 0.001). There was no significant difference in the percentage of low-affinity interleukin-2 (IL-2) receptor-positive CD4⁺ lymphocytes in lung and blood (9 ± 3% versus 13 ± 2%). Analysis of the DNA synthetic cell cycle showed that ∼5% of blood CD4⁺ lymphocytes and ∼25% of CD4⁺ PIL were in S/G₂/M. Compared to homologous blood T cells, purified PIL displayed enhanced proliferative responses to IL-2 and diminished responses to the lectin phytohemagglutinin. Lectin-stimulated PIL showed greater secretion of interferon-γ and IL-2 than did blood lymphocytes. We conclude that T lymphocytes isolated from human lung parenchyma display the surface immune phenotype and functional activities of sensitized T “memory” cells. We hypothesize that compartmentalization of sensitized T cells in the lung parenchyma may provide a mechanism by which circulating immune T cells may be sampled for restimulation by inhaled antigens.