It is well known that thymic hormones can counteract immunodepression due to radiation therapy, preventing and reducing the severity and the number of myelotoxic and hematologic reactions. We tried to confirm these findings in a controlled multicenter clinical study involving 1,060 patients undergoing radiation therapy (580 treated with thymopentin 50 mg s.c. every other day, after irradiation and for at least 6 cycles of 4 weeks each, and 480 control patients). Highly statistically significant results (to the ANOVA test) were obtained in the protection against radiation-induced leukopenia in the treated group; furthermore, the treated patients had a marked reduction (p = 0.003 chi 2 test) in the early delayed reactions to irradiation, namely in the upper aero-digestive tract. In general, we observed a better, but not statistically significant recovery of the blood parameters, lymphocyte subsets and skin tests in the treated group versus the control group. Both of the treated groups showed the same trend for Karnofsky performance status and body weight. The local and general protection provided by thymopentin against the reactions to irradiation could be advantageously used for the administration of higher doses of radiation therapy.