Hematopoietic stem cell toxicity of the murine monoclonal antibody 7.2, recognizing Ia-like antigens on canine cells, was tested in an autologous bone marrow transplantation model. Dogs were given 9.2 Gy of total body irradiation followed by the infusion of autologous marrow treated by one of two methods to remove Ia+ cells. In six dogs, the marrow cells were pelleted, treated with antibody 7.2 (1:1,000) and rabbit complement (1:4), resuspended in culture medium, and infused. All six dogs had prompt and sustained engraftment surviving greater than 26 days. Indirect immunofluorescence showed, however, that the depletion of Ia+ cells was incomplete. Four dogs received marrow cells first separated by density gradient centrifugation and then treated with an excess of antibody 7.2 and two cycles of undiluted rabbit complement. None of these dogs, surviving 17 to 22 days, had sustained engraftment. With antibody 7.2 used as the marker, only one dog had detectable residual Ia+ cells (0.9%) after treatment. Dogs receiving marrow cells obtained by density gradient centrifugation without additional manipulation, or with subsequent treatment with complement only or with complement and an antibody (DT-2) directed at a subpopulation of T cells, engrafted promptly and completely. We conclude that Ia+ bone marrow cells are essential for the successful engraftment of transplanted marrow in dogs.