SUMMARY The serological properties of γ-globulin of a horse anti mouse thymocyte serum (HAMTG) were studied using mixed agglutination (MA) with L cells and cytolysis in agar (CA) of murine thymocytes. In both tests, HAMTG gave strong positive reactions. Absorption by L cells, thymocytes, and liver cells decreased the MA activity of HAMTG considerably and to a similar degree. Thymocytolytic activity was readily removed by thymocytes. It was also removed, but less readily, by spleen cells and it was hardly affected by absorption with L cells or liver cells. Antibodies specific for lymphoid cells demonstrated by these experiments were further investigated in CA. Evidence was presented that some of these antibodies reacted with antigens identical with, or closely related to, previously described mouse-specific lymphocyte antigens (MSLA). Other antibodies were directed against the murine lymphoid alloantigen Ly-A.2. Evidence was also presented that the immunosuppressive potency of HAMTG was not affected by the removal of antibodies directed against nonlymphoid cells.