Twelve dogs with spontaneous malignant lymphoma in chemotherapy-induced remission were treated with total-body irradiation (TBI) and transplantation of autologous peripheral blood mononuclear cells collected following chemotherapy-induced expansion of the stem cell pool. Four animals (33%) died of transplant-related complications, five (41%) relapsed, and three (25%) are long-term disease-free survivors. Recovery to 1000 white cells/mm3 occurred by day 16 and dogs no longer required platelet transfusions by day 37. With the exception of delayed platelet recovery these results are virtually identical to those previously reported using autologous bone marrow transplantation for canine lymphoma. This study therefore suggests that autologous peripheral blood mononuclear cells collected following chemotherapy-induced expansion of the stem cell pool may offer a realistic alternative to autologous marrow transplantation in patients with malignant lymphoma for whom autologous marrow is not available.