In an attempt to examine tumor specific antigens, the crude suspensions of Ehrlich ascites tumor cells and the mouse muscle homogenates were treated by fluorocarbon. Antisera against the fluorocarbon treated antigens were prepared in rabbits. The antisera were tested for the presence of antibodies against the homologous and heterologous antigens by complement fixation test. The anti-tumor serum contained antibody against the homologous antigen, but no detectable amount of antibody against the muscle antigen. The anti-muscle serum, conversely, possessed an antibody against the muscle antigen, but showed no antibody against the tumor antigen. By means of the complement method of immunofluorescence, Ehrlich tumor cells were specifically stained with the anti-tumor serum. The staining titer, as expressed by its final effective dilu-tion, was comparable to those demonstrated in the in vitro complement fixation test. Furthermore, it was found by immunofluorescence that the anti-tumor serum stained the cells of ascites of sarcoma 180 and those of leukemic ascites tumor SN 36 in mice, but not the peritoneal non-tumor cells obtained from normal mouse.