We have partially purified a lymphokine, costimulator, which is necessary to induce mitogenesis in mouse thymocytes in vitro. Costimulator is released from mouse leukocytes exposed to Con A for 12 to 18 hr. It has been purified more than 100 X by gel exclusion chromatography and isoelectric focusing. Thymocytes from CBA/J mice respond to the mitogenic lectin Con A only if the costimulator concentration is above a certain level. Culturing such cells with Con A at a density below 1 × 106 cells/ml produces costimulator concentrations too low for mitogenesis. This system has been developed into a quantitative assay for costimulator, to monitor purification, recovery, and biologic activity in various methods of molecular characterization. The activity is trypsin sensitive, and has a buoyant density characteristic of protein or glycoprotein. However, for a protein, it is relatively heat stable. Its m.w., established by carrying out sedimentation, gel filtration, and buoyant density measurements, is 30,500, and its frictional coefficient is 1.45. Costimulator purified by isoelectric focusing is active at 10-10 M or lower in tissue culture.