The efficiency of a gravity-dependent autotransfusion system (ATS) for filtering tumor cells from blood was examined under laboratory and clinical conditions. In laboratory studies, reconstituted whole blood was seeded with known numbers of cultured A431 human vulva carcinoma cells then filtered through the ATS. Cancer cells were recovered from blood samples taken before and after filtration using a density gradient centrifugation procedure, and identified by immunocytochemical techniques, based upon their cytoskeletal intermediate filament proteins. It was estimated that 55%-76% of the A431 cells were retained by the ATS filters, and of those cancer cells that passed through the ATS, 62% suffered lethal trauma and the remainder showed morphologic damage. In clinical studies, blood was harvested from the surgical site during resection of primary cancers from three patients. In agreement with the laboratory studies, 50%-68% of tumor cells were filtered by the ATS from blood harvested during surgery. These results are discussed in terms of metastatic inefficiency and the potential for use of autotransfused blood salvaged during cancer surgery.