Mice hosting a heterogeneous human colon xenograft tumor produced by subcutaneous injection of the DLD-1 tumor cell line were treated either with x irradiation alone, with mitomycin C alone (4 mg/kg), or with x irradiation given two hours after intraperitoneal injection of mitomycin C (4 mg/kg). Radiation alone produced a dose dependent delay in the time needed for tumors to regrow to twice their size at the time of irradiation, and in the mice receiving mitomycin C plus x irradiation, an additional growth delay equivalent to that produced by 3-3.5 Gy of X rays was seen at all X ray dose levels. As the DLD-1 tumor xenografts do not appear to possess a significant hypoxic fraction, we conclude that the two agents are acting in a simple additive cytotoxic manner by the killing of oxic tumor cells.