gamma-Radiolysis, with doses less than 1 kGy, of aqueous solutions of disulphides, disulphide-proteins or thiols leads to the generation of stable products, capable of stimulating the catalytic reduction of Fe(III)-cytochrome-c by unirradiated glutathione, and by other thiols. The stimulatory activity fades within 20-60 min in the case of irradiated thiols, but there was little loss of this activity when irradiated solutions of disulphides or disulphide-proteins were stored at 4 degrees C for days. Disulphides (e.g. cystamine) are mainly activated by .OH radicals, disulphide-proteins (e.g. alpha-chymotrypsin) mainly by e-aq, and thiols (e.g. cysteine) by virtually all water radicals. The radiolytic activation, which is only partially prevented by oxygen, can be attributed to the generation of trace amounts of higher sulphides and persulphides (RSSSR, RSSSSR and RSSH). Such species are known to stimulate Fe(III)-cytochrome-c reduction by glutathione in a chain reaction (Massey et al. 1971). The radiolytic stimulation of reductive catalytic activity of thiols and disulphides may play a role in irradiated biological systems, and might be exploited to identify irradiated proteins with Fe(III)-cytochrome-c as detector.