Cytolysis of nucleated cells incorporated into agar gel was studied using hetero- and isoantibodies. Evidence was presented that IgG and IgM antibodies may participate in these reactions. Studies on the lytic activity of rabbit anti-rat sera showed that antibodies inducing lysis of rat thymus cells are directed against species-specific cell surface antigens. Evidence was presented that some of these antigens are shared by nucleated cells and erythrocytes whereas other antigens are present on nucleated cells only. Tests with rabbit anti-sheep erythrocyte serum demonstrated that Forssman antibodies elicit lysis of nucleated cells originating from Forssman-positive species. It was also shown that infectious mononucleosis sera are capable of inducing lysis of sheep and bovine nucleated cells. In studying human renal transplantation sera, cytolysis of kidney cells was induced by sera obtained from recipients after removal of rejected grafts or pretransplantation sera of recipients who rejected hyperacutely their grafts. These results were consistent with those observed in previous studies by means of mixed agglutination.