In mammals the major portion of cyanide is converted to thiocyanate by the liver enzyme thiosulfate sulfur transferase (TST) (rhodanese). We have found a much reduced activity of this enzyme in liver biopsies from two affected males of a family with Leber's hereditary optic atrophy and in two isolated cases of the same disease, (compared to liver biopsies from controls or liver samples obtained at autopsy). In one of the patients we studied the effect of a 3-day thiosulfate infusion. The urinary excretion of thiocyanate which was low prior to the infusion was raised during the thiosulfate treatment; in a healthy control person the same thiosulfate infusion did not alter the thiocyanate excretion rate. This suggests that cyanide detoxification which is suboptimal in patients with Leber's disease may be increased by thiosulfate infusion.