The effect of 12 weeks of cyclosporin A (CyA) (7 mg/kg) on the survival of vascularized osteochondral allografts between rat strains-Dark Agouti (DA donor) and Lewis (recipient)-was examined up to 6 months after grafting. Grafts were assessed by India-ink infusion to examine their microcirculation, and by quantitative histology. Isografts (Lewis to Lewis) survived at least 25 weeks, but displayed progressive deterioration due to their non-weight bearing position. Rejection controls (allografts with no immunosuppression) showed rejection within 2 weeks. Allografts in immunosuppressed hosts remained healthy for the 12-week period of immunosuppression, but deteriorated progressively during the ensuing 14 weeks, particularly in the muscle, marrow, and growth plates. Graft repopulation by host cells was assessed by transferring grafts into fresh non-suppressed allografts hosts, following 12 to 26 weeks in the first, immunosuppressed host. All grafts were rejected rapidly following the second transfer, indicating that little or no cellular repopulation of the graft had occurred while in the first host.