Experiments were designed to assess 1) relative immunity after adoptive transfer of spleen cells or serum from tumor-bearing mice to untreated syngeneic mice, and 2) the degree of tumor-specific transplantation immunity imparted by cells or serum relative to tumor size and the presence of metastases in the donor at the time of spleen cell or serum transfer or after in situ necrosis of the primary tumor by cryosurgery in a CDF1-sarcoma system. Viable lymphoid spleen cells or serum from normal mice had no effect on tumor growth. Serum from mice that had large tumors and gross metastases induced a protective effect similar to that found after cryosurgery of the primary tumor. Serum from mice with small tumors and spleen cells from animals bearing either small or large tumors failed to induce immunity consistently. In no instance did serum from tumor-bearing mice induce enhancement of tumor growth.