The effect of heritable LFA-1 deficiency on T lymphocyte function was measured. After primary mixed lymphocyte stimulation, all six patients studied showed diminished allospecific T lymphocyte cytolytic and NK activity as compared with kindred and normal controls. MLR and mitogen-induced proliferative responses were consistently depressed. LFA-1-deficient, EBV-transformed B cell lines were poor stimulators of T cell responses. Primary cytolytic responses by lymphocytes from severely LFA-1-deficient patients (less than 0.2% of normal surface expression) were consistently more profoundly depressed than those by lymphocytes from moderately deficient patients (about 5% of normal surface expression). These results demonstrate the importance of LFA-1 in lymphocyte function. After repeated MLR restimulation, proliferative and cytolytic capacity improved and CTL lines could be established from all patients. Cytolysis by lines from one but not a second severe patient, and by four of four moderate patients, was inhibited by anti-LFA-1 MAb, and at 10-fold lower concentrations than required for inhibition of killing by control CTL lines. The locus of inhibition was on the target cell for the severely deficient CTL line, and on both the target and effector cells for moderately deficient CTL lines. In contrast, the locus of inhibition for normal CTL is on the effector cell. These findings show that LFA-1 can participate bidirectionally in cell interactions. The in vitro results are discussed in terms of the clinical findings in patients.