Mouse thymocytes activated the alternative complement pathway of mouse serum in the presence of heated fetal calf serum. The activation required C3 from the fetal calf serum but was independent of antibody either in the murine or bovine serum. No other murine cells tested, including erythrocytes, peripheral blood lymphocytes, lymph node cells, spleen cells, and various cultured cell lines, activated the alternative complement pathway as effectively as thymocytes. In addition, sera from species other than cows could not substitute for fetal calf serum. The C3 deposited on thymocytes was in the form of both C3b (immune adherence positive) and C3bi (conglutinable). We propose that the basis of activation in this system is the specific protection of bovine C3b on mouse thymocyte surface.