The capacity of human peripheral blood lymphocytes to suppress DNA synthesis of other lymphocytes was studied in an assay consisting of two steps: firstly, activation by Con A during 24 hr followed by alpha-methylglucoside and mitomycin treatment; secondly, incubation of these Con A-activated 'suppressor' cells with autologous responder cells and stimulants, or incubation with allogeneic responder cells. The results were compared with cells similarly treated but not incubated with Con A. If alpha-methylglucoside treatment is omitted, stimulation of T and non-T cells occurs by Con A bound to the Con A-activated cells. Con A is especially bound to non-T lymphocytes and even gives a T cell-independent proliferation of non-T cells without differentiation to plasma cells. With alpha-methylglucoside treatment, 'suppressor' cells, activated by high Con A concentrations, are able to suppress DNA synthesis of autologous lymphocytes stimulated by allogeneic cells or soluble antigens to about 50%. In a one-way MLC, in which the cell suspension containing the suppressor cells is also used as a stimulator cell suspension, a similar suppression was observed. Suppression of DNA synthesis was correlated with suppression of proliferation without evidence of cytotoxicity.