Donor leukocytes in therapeutic blood components are implicated in transfusion-related complications ranging from alloimmunization to graft-versus-host disease (GVHD) to viral transmission and reactivation. To further characterize the kinetics of donor leukocyte clearance after allogeneic transfusion, we developed allele-specific polymerase chain reaction (PCR) assays directed at a single-copy Y chromosome gene and HLA class II alleles. These assays enable sensitive detection and quantitation of donor leukocytes at concentrations ranging from one cell to greater than 1,000 cells per 125 microL of recipient blood. When applied to serial samples from five consecutive orthopedic surgery patients who met study criteria, we observed 99.9% clearance of donor leukocytes over the initial 2 days posttransfusion, followed by a transient, 1-log increase in circulating donor leukocytes on days 3 to 5. This phenomenon was reproduced in a canine transfusion model, where the transient donor leukocyte expansion phase was prevented by gamma irradiation of donor blood, and was not observed after transfusions into alloimmunized dogs. We hypothesize that this transient increase in circulating allogeneic donor cells represents one arm of an in vivo mixed lymphocyte reaction, with activated donor T lymphocytes proliferating in an abortive GVHD reaction to HLA-incompatible recipient cells. Further investigation of this phenomenon should provide insight into the mechanisms involved in donor-recipient leukocyte interactions posttransfusion and the relationship of these interactions to leukocyte-induced complications.