CD4, a 55-kd cell surface glycoprotein, binds to class II major histocompatibility complex (MHC) (Ia) antigens and functions as a coreceptor for the T cell antigen receptor (Ti alpha beta)-CD3 complex. We have observed that critical elements of the T cell antigen multireceptor complex, including Ti alpha beta, CD3, CD4, but not CD8, were diminished on CD45RO+ pulmonary T lymphocytes but not CD45RO+ peripheral blood T lymphocytes (PBL). Epitopes mapping from the first (D1) to the fourth (D4) extracytoplasmic Ig-like domains of CD4 were expressed to a lesser degree on pulmonary T cells than on PBL (P = 0.002). CD4 expression on pulmonary T cells did not increase after 72 hours of ex vivo culture in complete medium but was restored toward control levels by stimulation with phytohemagglutinin, anti-CD3, or interleukin-2. CD4 mRNA isolated from lung T cells and PBL co-migrated on Northern blots and the total levels of CD4 mRNA were comparable, suggesting that diminished CD4 expression by pulmonary T cells might reflect a posttranscriptional change. To determine whether CD4bright T cells convert with mitogen stimulation to CD4dim cells, PBLs were stimulated with immobilized anti-CD3, anti-CD4, or a molecularly engineered anti-CD3:CD4 bispecific monoclonal antibody and the ratio of the CD4:CD3 mean fluorescence staining intensities was calculated at days 3 and 13. The CD4:CD3 ratio decreased primarily for cells stimulated with anti-CD3:CD4, suggesting that co-ligation of CD3 and CD4 is required for the generation of CD4dim T cells. We conclude that diminished Ti alpha beta-CD3:CD4 expression is a characteristic of T cells in lung that is not shared by peripheral blood T cells in vivo, and speculate that this change reflects T cell activation in a millieu of limited interleukin-2 availability.