We analyzed the effect of 2 hybridoma monoclonal antibodies (Mab), D1-12 and D4-22, with specificity for common determinants of human Ia molecules, on the mixed lymphocyte culture (MLC) response. The results show that addition of either of the 2 Mab as late as day 3 after the onset of the culture completely inhibits the proliferative response generated in MLC. Because the antigenic determinants recognized by the 2 Mab that were used in this study have been shown to belong to distinct Ia molecules, it appears the inhibitory effect observed in MLC containing such Mabs cannot be explained simply by the masking of Ia molecules on the stimulator cell population. In agreement with previous studies by other investigators, treatment of a leukocyte population with the cytolytic D1-12 Mab plus complement strongly reduced its ability to stimulate in MLC. More importantly such a treatment also decreased the ability of a leukocyte population to respond in MLC. In the latter case, the inhibitory effect appears to be directed against T cells since highly purified E-rosetting cells treated with D1-12 plus complement were unable to respond in MLC. The possible implications of these results are discussed.