In this study we report on measurements to determine the binding constants of immunoglobulins to tumor cells. IgG from normal and from tumor-bearing mice was studied. It was found that IgG preparations from SEYF-a (a syngeneic polyoma-virus-induced ascites tumor)-bearing mice contain at least two distinct populations of molecules which differ from one another with respect to their binding constants to the tumor cells. One population is characterized by a high binding constant to SEYF-a cells, while the second population reveals a much lower binding constant to these cells. Normal mouse IgG preparations also bind to tumor cells, however, their binding constant is low. Soluble antigens extracted from the corresponding tumor cells were able to inhibit specifically the binding of IgG molecules originating from tumor-bearing mice, which exhibited the high binding constants to the tumor cells. Antigens extracted from non-related tissues did not affect the binding patterns of IgG to the tumor cells. Moreover, IgG prepared from SEYF-a-bearing mice exhibited a high binding constant only when interacted with the corresponding tumor cells, but not when interacted with non-corresponding ones. These results suggest that the IgG fraction with the high binding constant is composed mainly of antibodies to surface antigens of tumor cells. Determination of binding patterns of anti-tumor antibodies to the corresponding and to non-corresponding tumor cells could prove useful as an additional method for determinations on the degree of serological relations between various tumors and for further characterization of anti-tumor antibodies.