The aim of these studies was to measure the extent of renal damage after fractionated irradiation in combination with cis-diamminedichloroplatinum(II) (c-DDP) and to determine whether there was any drug-induced inhibition of repair of sublethal X-ray damage. Fractionated irradiations were given either in a short overall treatment time of 2 days or in a total time of one month, in which irradiations were given in the first and fourth weeks only. A single dose of c-DDP was given before the first X-ray dose for treatments in a total time of 2 days and before the first X-ray dose of each week for the split-course treatments. Renal function was measured at monthly intervals (from 18 to 39 weeks after the start of treatment) by clearance of 51Cr-EDTA. The renal damage was always worse after combined treatments (X + c-DDP) than after X-rays alone. A comparison of radiation dose-response curves with and without c-DDP yielded Dose Enhancement Factors (DEFs) of 1.2 to 1.4 for all fractionation schedules, with no trend for an increase in DEF with increasing number of fractions. A Linear-Quadratic (LQ) analysis of the data demonstrated that there was no change in the alpha/beta value for renal radiation damage for the combined treatments compared with X-rays alone (alpha/beta = 2-3 Gy). These results suggest that the increased renal damage which occurred after combined X + c-DDP was due to independent, additive toxicities and not to radiosensitization or inhibition of X-ray repair by the drug.