In order to trace genetically the source of fallout endothelialization on arterial grafts, six beagle dogs with successful autologous bone marrow transplantation received composite tandem aortic grafts with an isolated, totally impervious Dacron graft and a porous Dacron graft for 12 weeks. For impervious segments, five of 12 fresh tissue samples were Factor VIII/von Willebrand factor + (FVIII/vWF) and seven had faint or negative signals; three of the FVIII/vWF + samples had alpha-actin + smooth muscle cells. Polymerase chain reaction (PCR) study showed eight had a pure donor DNA genotype and four had donor/host mixed, with the donor predominant. Of 12 AgNO3-stained samples, 11 showed pure donor type and one had donor/host mixed, with the donor predominant. For porous segments, all 12 fresh samples had positive flow surface FVIII/vWF and alpha-actin cells. PCR showed all these samples and all 12 AgNO3-stained samples had donor/host mixed type, but the host pattern was predominant. Porous graft healing appears to involve both cellular fallout and tissue ingrowth, and bone-marrow-derived cells may be a source for fallout.