Interleukin 1 (IL-1) is a radioprotector of bone marrow and is cytotoxic to some tumor cells. This investigation examines these two properties in the same host animals and gives evidence of radioprotection against localized x-irradiation of the head and neck region. By LD50 analyses, recombinant human IL-1 (100 ng/mouse, approximately 3 micrograms/kg) was found to be radioprotective against whole-body irradiation for both C3H/Km and C57BL/Ka mice. The combined potency ratio for the two strains was 1.07 (95% confidence limit: 1.02-1.12). It was also radioprotective against the injury leading to acute lethality resulting from localized head and neck irradiation of C3H/Km mice; 100 ng of IL-1/mouse produced a potency ratio of 1.05 (95 confidence limit: 1.03-1.07). However, two tumors that originated in C3H/Km mice, RIF-1 and SCCVII, showed neither in vitro nor in vivo response to IL-1. Also, there was no IL-1-induced reduction in in vivo growth of the RL 12NP lymphoma in C57BL/Ka mice.