We have previously described a monoclonal antibody (MAb), M7/14, which blocks a variety of T cell functions, including CTL-mediated killing, the mixed lymphocyte response, and antigen-specific proliferation. The antigen defined by M7/14 has been designated lymphocyte function-associated antigen one (LFA-1). In this report, LFA-1 has been studied as to cell distribution, surface abundance, structure, and in comparison to other CTL surface antigens, LFA-1 is expressed on lymphoid cells of both the T and the B lineages and on a large fraction of bone marrow cells, but not on exudate macrophages or nonlymphoid tissues. T cells express more LFA-1 than B cells, both in the unstimulated and stimulated states. Compared with unstimulated spleen cells, cytolytic T lymphocyte cell preparations (CTLP) and Con A blasts, but not LPS blasts, show increased LFA-1 expression relative to H-2, and for T cell-containing populations, Lyt-2. M7/14 MAb binds to about 1.5 X 10(4) and 7 X 10(4) LFA-1 sites per average spleen cell or CTLP cell, respectively. M7/14 Mab binds to cTLP in quantitites of 2.5-fold and ies 10.4-fold less than H-2 and Thy-1 Mab, respectively; since the latter have little or no effect on CTL function, inhibition of killing by M7/14 MAb is specific for the LFA-1 surface site. M7/14 MAb and a blocking Lyt-2 MAb are bound in similar quantities of CTLP. LFA-1 is a glycoprotein and consists of 2 noncovalently linked polypeptide chains of 180,000 and 95,000 Mr. The same molecular species as on CTL is present on other T cells and on B cells. The molecular structure and cell distribution of LFA-1 clearly distinguishes it from Lyt-2,3, Ly-5, T145, and T11, which were previously suggested to be either associated with the function of and/or present on the surface of CTL.