Signals required for expression of HLA-DR (DR) antigen in phytohemagglutinin (PHA)-activated human peripheral blood T cells were examined. T cells were purified by a four-step procedure, which included depletion of glass-adherent cells, 53% Percoll gradient centrifugation, nylon wool column passage, and treatment with mouse monoclonal antibodies directed to human HLA-DR antigen and Leu M1 antigen plus complement. Purified T cells responded poorly to PHA but with the combination stimuli of PHA and recombinant human interleukin 2 (rIL-2), resting T cells proliferated as well as T cells cultured with 10% monocytes and PHA. But well proliferated T cells in the absence of monocytes expressed very poor DR antigen after 7 to 8 days of culture. DR expression of T cells was restored by the addition of 10% monocytes. Allogeneic monocytes also helped proliferative responses of PHA-activated T cells but did not help the expression of DR antigen. These results suggested that signals required for T cell proliferation (PHA and rIL-2) were not sufficient for DR expression in this system and further monocytes were essentially required in a HLA-restricted manner. In the next experiment, we examined the role of membrane molecules in monocytes for transmission of signals that induce activated T cells to express DR antigen. Autologous monocytes were fixed with 1% paraformaldehyde and added to T cells in the presence of PHA and rIL-2. Fixed monocytes could help DR antigen expression of PHA-activated T cells as well as viable monocytes. But when fixed monocytes were pretreated with anti-HLA-DR monoclonal antibody, they could not help DR expression of T cells any longer. These results suggested that for the expression of DR antigen, PHA-activated T cells had to first recognize self DR antigen expressed on the surface of monocytes before proliferation occurred.