Dogs were given single dose or fractionated total body irradiation (TBI) and autologous marrow grafts to prevent death from myelosuppression. Acute and delayed non-marrow toxicities were compared. Fifty-six dogs were given single dose TBI at 2.1 (n = 13), 5 (n = 12), 10 (n = 15), or 20 (n = 16) cGy/min. Acute radiation toxicity and mortality was related to the exposure rate; radiation doses resulting in 50% mortality at 7 days (LD 50/7) at 2.1, 5, 10 and 20 cGy/min were 1,692, 1,499, 1,261, and 1,056 cGy respectively. Fifty-three dogs were given fractionated TBI, 200 cGy three times a day with 6-hour intervals at 2.1 (n = 13), 5 (n = 9), 10 (n = 13), or 20 (n = 18) cGy/min. The LD 50/7 at the four exposure rates were 1,628, 1,470, 1,184, and 1,320 respectively. Thus, for exposure rates of 2.1, 5, and 10 cGy/min, the tolerated doses were comparable for single dose and fractionated TBI. At 20 cGy/min dose fractionation appeared to offer some advantage, although this fractionation effect in part may have been due to random variation with small numbers of dog treated. Following recovery from the immediate TBI-related toxicity, eight dogs given single dose and four dogs given fractionated TBI died, generally from infections, 8-30 days following transplantation. There was a striking difference in regards to long-term survival dependent upon the TBI regimen. Among dogs given greater than or equal to 1,000 cGy of TBI and alive 30 days after transplant only 1 of 18 given single dose TBI became a long-term survivor compared to 19 of 22 given fractionated TBI. Causes of death included pancreatic fibrosis, malnutrition, hepatic failure, and a generalized wasting syndrome. All 5 dogs given a single dose of 800 cGy (at 20 cGy/min) became long-term survivors. Fourteen dogs were given increments of 150 cGy at 7 cGy/min every 3 hours for total doses of 1,500-2,400 cGy. The LD 50/7 was approximately 1,900 cGy. All 6 dogs alive at 30 days became healthy long-term survivors. Four dogs were given increments of 600 cGy at 2.1 cGy/min every 48 hours for a total dose of 1,800 cGy. All 4 dogs became long-term survivors. In conclusion, exposure rate and total dose are the most important parameters for acute toxicity associated with TBI. The effect of dose fractionation is minimal at low exposure rates and appears to be dependent also upon increment size and fractionation interval.(ABSTRACT TRUNCATED AT 400 WORDS)