The main source of donor DNA in recipients of allograft are "passenger" cells. It is claimed that they are responsible for the posttransplantation microchimerism and prolongation of allograft survival. We have observed that besides cellular microchimerism, donor DNA can be found in the recipient tissues at the time of rejection of the allograft. In this study, we provide evidence for the presence in the recipient of both DNA in "passenger cells" and free DNA in tissues at the terminal stage of rejection. Male BN (RT1 n) rat heart or skin was transplanted to female LEW (RT1 l) rats followed by a vascularized bone marrow in a hindlimb transplant. In another group, heart and skin were transplanted followed by immediate i.v. infusion of donor-type bone marrow cells. CsA was given in a dose of 17 mg/kg body weight for 30 days, then the rats were followed up until day 100 unless rejection occurred earlier. LEW blood, spleen, mesenteric node and bone marrow cells were stained with moAb OX27 specific for BN but not LEW. Genomic male DNA was isolated and amplified with SRY oligonucleotide. At day 30 and day 100 cellular microchimerism was detected in blood, spleen, nodes and bone marrow cells. Donor DNA was detected in recipient skin, liver and heart extracts, as well as lymphoid organs, at the time of rejection of allograft, but not when the rats were maintained on CsA. Taken together, donor DNAwas detected in recipient tissues at the time of heart or skin rejection. It appeared to be released from cells of rejecting grafts and not from "passenger" cells, representing only a minor cellular mass compared with the graft.