Spontaneous metastasis formation was studied in thymectornized andlor sublethally irradiated syngeneic C57BL/6/ mice of a highly antigenic MCA-induced The possible influence of immune reactions on spontaneous metastasis formation of two MCAinduced sarcomas was previously tested in bonemarrow-substituted radiation and thymectomized radiation chimeras of C57BL/6J mice (Boeryd and Suurkula, 1973). No effect on the spread of the sarcomas was noted. This suggested that the influence of immune reactions on the spread of the two sarcomas was negligible, or that the activity of the presumed counteracting immunological mechanisms (Klein, 1972) was equally depressed.The aim of the present work was to test these possibilities further by studying the effect of less extensive and varying degrees of immunosuppression on the spread of one of the previously used sarcomas. The experimental model was the same except that sub-lethal, instead of lethal, irradiation was used. This treatment depresses the humoral antibody response to sheep red blood cells (SRBC) and the homograft reaction in various strains of mice (Koller and Doak, 1960;Micklem and Brown, 1961; Rosenthal, 1969) and recovery is dependent upon the pre-