Intraoperative autotransfusion is contraindicated in tumor surgery because of the danger of tumor cell dissemination. We have tested the elimination of tumor cells in blood by irradiation for safe retransfusion. Tumor cells of various origin were mixed to washed red blood cells from volunteer blood donations. The blood was irradiated with 50 Gy. After isolation of the tumor cells by density gradient centrifugation they were tested for colony formation. While with different tumor cell lines (n = 12) 10 cells were sufficient to yield several colonies, as many as 10(10) cells did not result in any colony after irradiation of the blood. Similar results were obtained with cells cultured from blood salvaged during tumor surgery (n = 3), and with tumor cells prepared from various carcinomas (n = 10). Flow cytometric DNA analysis showed the irradiated cells in mitotic arrest. None of these cells had residual DNA metabolism expressed as incorporation of BrdUrd. We were able to demonstrate a rate of reduction in dividing cells of up to 10(9). With the typical irradiation sensitivity of tumor cells, with D0 values between 1 and 2 Gy, a dose of 50 Gy results in an effective log 12 reduction, sufficient for safe elimination of tumor cells found in shed blood. No adverse effects of the gamma-irradiation on the blood cells are to be expected, especially since the blood is retransfused without storage.