Significant morbidities are associated with the routine administration of blood products. Although the exact etiology of these complications may be unknown, many are thought to arise from the incidental cotransfusion of "donor" lymphocytes. We have developed an assay to detect small numbers of male white blood cells (WBCs) circulating in female patients who have received multiple blood transfusions using the polymerase chain reaction (PCR). Twenty female patients undergoing major surgical procedures were studied and received an average of 9.3 U of packed red blood cells (4.8 U from male donors) and 11.7 U of platelets (6.1 U from male donors). DNA was extracted from whole blood or peripheral blood buffy coats posttransfusion and PCR performed using oligonucleotides designed to amplify a segment within the repetitive Y-chromosome DYZ1 locus. Posttransfusion, 15 of 20 women showed evidence of circulating male WBCs for an average of 2.0 days (range, 1 to 6). We conclude that (1) DYZ1 PCR analysis is a useful approach for the detection of small numbers of circulating transfused male WBCs in female patients; and (2) circulating donor WBCs persist for a mean of 2.0 days in the majority of women receiving multiple transfusions. Future application of this technique may detect persisting or proliferating WBCs and lead to an improved understanding of common transfusion-related morbidities.