To elucidate its potential role in the framework of bone marrow transplantation, we studied the toxicologic and pharmacologic properties of high doses of the triepoxide derivate 1,2,4-triglycidylurazol (TGU) in a preclinical dog model. Dose-dependent and dose-limiting gastrointestinal toxicity occurred in a dose range between 40 and 75 mg/kg, with the lethal dose for 50% of animals (LD50) being estimated at 60 mg/kg. Severe and life-threatening hematologic toxicity developed at all dose levels examined but was generally reversible. The combination of TGU and total-body irradiation produced synergistic gastrointestinal toxicity, necessitating reductions of the TGU dose by 50% as compared with the single-agent dose. In contrast, the combination of TGU and high-dose busulfan resulted in no apparent nonhematologic synergistic toxicities. The immunosuppressive properties of TGU given in this combination enabled sustained histocompatible allogeneic marrow engraftment in three of four animals. The pharmacokinetics of TGU were not influenced by prior total-body irradiation or high-dose busulfan. We conclude that the myelotoxic, pharmacologic and immunosuppressive properties of high-dose TGU observed in this preclinical model seem to render the drug particularly suitable for use in regimens preparatory to bone marrow transplantation.