We have established an immunomagnetic separation procedure for the detection of circulating tumor cells in the peripheral blood based on the magnetic cell sorting (MACS) technique. In previous in vitro experiments, renal-cell carcinoma (RCC) cells were mixed with peripheral blood. In dilutions of 1:200 to 1:107 tumor cells per mononuclear blood cells, an average recovery rate of 84% of tumor cells was determined. In our study, 104 peripheral blood samples from 59 renal carcinoma patients were analyzed. MACS resulted in significant depletion of leukocytes, permitting a search for tumor cells on just 1 slide. Analyzing 8 ml of peripheral blood per patient, 19/59 RCC patients carried disseminated tumor cells (32%) in the range of 1 to 38 cells (median 8). Interestingly, for the cytokeratin-positive (CK+) patient group, we found a correlation between tumor cell number and grading (G2 vs. G3) and an increased number of CK+ patients with advanced tumor stage. MACS appears to be an efficient technique to detect disseminated tumor cells in peripheral blood.