N-(2-mercaptoethyl)-lt3-diaminopropane (WR1065) protects against radiation-induced cell killing and mutagenesis at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus in V79 Chinese hamster hing fibroblast cells. At a concentration of 4 mM, WR1065 was found to be effective in protecting against radiation-induced cell lethality only if present during irradiation, e.g., a dose modification factor (DMF) of 1.9. No protective effect was observed if the protector was added within 5 min after irradiation or 3 h later, e.g., DMFs of 1.0 and 1.1, respectively. The effect of WR1065 on radiation-induced mutation, expressed as resistance to the cytotoxic purine analogue 6-thioguanine (HGPRT), was also investigated. In contrast to the treatment-schedule dependence for protection by WR1065 against cell killing, this agent was effective in reducing radiation-induced mutations regardless of when it was administered. Following a dose of 10 Gy of 60Co γ-rays, the mutation frequencies observed per 106 survivors were 77 ± 8, 27 ± 6, 42 ± 7, and 42 ± 7 for radiation only, and WR1065 present during, immediately after, or 3 h after irradiation. These data suggest that although a segment of radiation-induced damage leading to reproductive death cannot be modulated through the postirradiation action of WR1065, processes leading to the fixation of gross genetic damage and mutation induction in surviving cells can be effectively altered and interfered with leading to a marked reduction in mutation frequency.