Blood transfusion prior to renal transplantation improves subsequent renal allograft survival, and some experimental studies have shown that allogeneic transfusion significantly increases growth of subsequently transplanted tumors. The aim of the present study was to examine the influence of the timing of transfusion on tumor growth. Six experiments were performed, each with different intervals between blood transfusion and inoculation of mice with an immunogenic B16 melanoma cell line. Each experiment contained three groups of C57 BL/6J mice which received intravenously normal saline, syngeneic blood, or allogeneic blood. Allogeneic transfusion 7 and 10 days before, but not at the time of, tumor inoculation produced a significant increase in tumor growth. Allogeneic transfusion within the first 10 days after tumor inoculation was associated with larger and heavier tumors than those observed in animals treated with saline or syngeneic blood, although the differences were not always statistically significant. Multiple transfusions after tumor inoculation and transfusions in animals bearing well-established tumors significantly increased tumor growth. The study shows that growth of a transplantable immunogenic murine tumor can be increased by prior allogeneic transfusion and, perhaps more importantly, by allogeneic transfusions given after the tumor has become established.