The anterior cruciate ligament of twenty-five adult dogs was replaced using fresh or deep-frozen patellar-tendon allografts. The morphology of these transplanted allografts was then evaluated using routine histological studies and a vascular-injection (Spalteholz) technique at various intervals from two weeks to one year postoperatively. The fresh patellar-tendon allografts incited a marked inflammatory and rejection response which was characterized by perivascular cuffing and lymphocyte invasion. Deep-frozen patellar-tendon allografts appeared to be benign within the joint and underwent alterations that were comparable with those observed in autogenous patellar-tendon grafts. These included avascular necrosis followed by revascularization and cellular proliferation. At one year, the gross and histological appearance of the patellar tendon allograft resembled that of a normal anterior cruciate ligament.