Monoclonal antibodies (MoAbs) were generated against cells or cell membrane glycoproteins of a human T-cell line, HPB-ALL. Five, designated MCN 1, 3, 12, 19, and 29, were found to be specific to helper/inducer T-cells; they gave a positive membrane staining to approximately 48% of peripheral blood lymphocytes and 84% of thymocytes and these proportions did not change upon costaining with Leu 3a, a known anti-helper/inducer T-cell MoAb. Furthermore, their reaction pattern with a panel of human lymphoid cell lines was identical to that of Leu 3a. A reciprocal binding blocking test showed that the epitopes reactive with the MCN MoAbs are divided into three separate clusters. The MCN 3- and Leu 3a-reactive epitopes formed a cluster and they appeared to be the same epitope. This cluster was well separated from that represented by the MCN 1-reactive epitope. The MCN 12-, 19-, and 29-reactive epitopes could be assigned to a third cluster. MCN 12 and 19 were probably toward the same epitope. A sequential binding test indicated that the three epitope clusters reside on the molecule carrying Leu 3a-defined epitope, i.e., the Leu3/T4 antigen. On the functional analysis, MCN 3 gave a profound inhibitory effect on T-cell proliferative response to MHC class II antigens, whereas other MCN MoAbs did not show any modifying effect on the T-cell function.