Concanavalin A (Con A) stimulation of T lymphocytes for 48 h induced a dose-dependent increase in the percentage of Tγ cells, and proliferation of these cells was dependent on pre-existing Tγ cells. Con A stimulation also induced a dose-dependent increase in suppressor cell activity as measured in a Con A/mixed lymphocyte culture assay. Removal of Tγ cells before stimulation with Con A did not influence suppressor cell activity, whereas depletion of Tγ cells after Con A stimulation significantly reduced the suppression. Treatment of T lymphocytes with monclonal anti-HLA-DR antibodies and complement significantly increased the percentage of Tγ cells and also increased the suppressor cell activity, both when the cells were treated before and when they were treated after Con A stimulation. Similarly, increased suppressor cell activity was also seen when Tγ cells were removed. The results indicate that Con-A-induced suppressor cells are recruited from both Tγ and Tnon-γ cells and also suggest that most of the Con-A-induced suppressor cell activity is exerted by HLA-DR-negative cells.