Con A is known to activate T cells to proliferation and the development of effector cells. Conflicting reports have been published as to the need for accessory cells in the T-cell response induced by Con A. We have found that the proliferative response in purified mouse spleen T cells induced by various doses of Con A requires different culture conditions and helper cells. The Con-A-induced response induced by optimal and high concentrations of the ligand requires the presence of either serum or adherent cells obtained from the peritoneal cavity or the spleen. For induction of a proliferative response by low doses of Con A both serum and helper cells must be present. The T-cell response to suboptimal concentrations of Con A is further characterized by the fact that the presence of an Ia-positive cell is required. Removal of Ia-positive cells from purified T cells results in a loss of the response of remaining cells to low doses of Con A but has less effect on the response induced by higher concentrations of the activating ligand. The possibility that distinct subsets of T cells are responding to low and high concentrations of Con A will be discussed.