BALB/c mice with syngeneic simian virus-40-derived tumors for 0–4 weeks were tested for their cellular immune capacity by a combination of in vivo and in vitro assays. Early tumor growth was associated with detectable cellular immunity, as measured by direct tumor challenge, the Winn test, and a modified microcytotoxicity assay. Animals with advanced tumors lost immunity detected by direct tumor challenge and the Winn test, whereas specific immunity detected by the microcytotoxicity assay persisted. These studies support previous concepts that the immune status of tumor-bearing mice is altered with increasing total tumor load. Additionally, the discrepancy between these assays performed in parallel may reflect functionally different components of the immune response to tumor-associated transplantation antigens.