We analyzed phenotypic and functional characteristics of intraparenchymal pulmonary lymphocytes in mice. As determined by flow cytometry, approximately 30% more T cells than B cells were found. Nearly all T cells bore the alpha beta TCR, most of which stained with either CD4+ or CD8+, although small numbers of double-negative T cells were present; the CD8+/CD4+ ratio was approximately 0.37. While practically all B cells bore surface IgM, and no IgA+ cells were found, and approximately more than 80% of the plasma cells produced IgA. Approximately half of the LPS-responsive cells produced IgA. Pulmonary B cell clonal precursors specific for the autoantigen, transferrin, were present in higher frequencies than those specific for the bacterial antigens, levan and Pseudomonas aeruginosa polysaccharide type I, or those producing antibodies against OVA. These results demonstrate that a distinct population of lymphocytes is present in the lungs.