Attempted nonspecific immunotherapy led to acceleration rather than retardation of tumor growth. Mice given injections of Freund's adjuvant were more susceptible to transplanted syngeneic Gross virus-induced leukemic cells when Freund's complete adjuvant was administered i.p. 0 to 7 days before or 1 day after tumor; thereafter, the adjuvant had no effect. Two serum-mediated phenomeana were demonstrated in vitro: (a) sera from mice immunized with Freund's complete adjuvant and tumor facilitated killing of tumor cells by peritoneal exudate cells from nonimmune mice; (b) sera from all mice with progressive tumor blocked the cytotoxicity of a xenogeneic tumor-specific serum. Certain sera produced both effects. However, sera that either blocked or facilitated tumor killing in vitro had no effect on the growth in vivo of transplanted tumor cells.