The mitogenic response of human T lymphocytes to graded doses of concanavalin A (ConA) has been measured by means of an MTT tetrazolium dye metabolic assay. Three groups of healthy subjects, representing children, younger adults and elderly persons, were investigated. It was shown that a typical bell-shaped course of the ConA dose-response curve is the result of a proliferative response to suboptimal concentrations of ConA and a toxic action of ConA at supraoptimal concentrations. The ascending part of the response curve reflects in its shape the regulatory interaction of responding cells. A decrease in suppressive functions is accompanied by a shift of this part of the curve to lower concentrations of ConA. By means of a mathematical model derived from enzyme kinetics, an attempt was made to quantify the suppressive functions from the course of the individual dose-response curve. It was found that after suitable data processing, suppression-related shape changes can be assigned to a single parameter. The value of this parameter as a diagnostic tool was tested in a study of the age dependence of human T lymphocyte responses to ConA. While the proliferative response decreased with age, the suppressive functions exhibited their maximum effect in the group of adults. Thus it could be demonstrated that ConA induced proliferative and suppressive responses are due to two different pathways which can be independently extracted from the dose-response curve.