The usefulness of total body irradiation (TBI) plus chemotherapy as a preparative regimen prior to bone marrow transplantation has been widely documented. However, the procedure can be highly toxic. Fractionated and low dose rate TBI has been said to enhance therapeutic ratio by increasing normal tissue tolerance and increasing leukemic cell kill. We report here the acute toxic effects and preliminary results on 2 consecutively groups of patients, treated with bone marrow transplantation (BMT) for leukemia or multiple myeloma, and conditioned by 2 TBI regimens. Group A patients received 10 Gy-Co-60 single dose of TBI plus 120 mg/kg of cyclophosphamide over a period of 2 days (8 Gy lungs). Group B received 12 Gy Co-60 of TBI in 6 fractions (2/day), (8 Gy lungs) plus 120 mg/kg of cyclophosphamide over a period of 3 days. The acute toxic effects recorded were similar in both groups. Only a 40% vs 0% (P = 0.02) incidence of parotiditis in groups A and B favors fractionation. Other results obtained to date are as follows: an incidence of interstitial pneumonitis of 39% and 31% (ns); relapses of 10% and 20% (ns), and mortality of 55% and 60% for each group respectively. An interesting finding was that IP was associated with acute grade II-IV graft vs host disease in 87% and 100% of cases of group A and B, respectively. We conclude that fractionated TBI is at least as effective as single dose TBI as a conditioning regimen; however, only randomized trials would allow definitive conclusions.