An enzyme method is described for the determination of micro-amounts of cyanide in aqueous and serum samples. The method was based on measurement of the absorbance at 460 nm of the iron(III) thiocyanate complex formed from thiocyanate produced in the enzyme-catalysed reaction. The enzyme rhodanese (E.C. 2.8.1.1) catalysed the reaction between the cyanide ion and thiosulphate with stoicheiometric formation of thiocyanate. Only a small concentration of enzyme was required. Linear calibration graphs were obtained in the range 0–20 mg l^–1 of cyanide in both aqueous and serum samples, with a detection limit of 0.1 mg l^–1. The precision of the method is better than 2.5% relative standard deviation at 5 mg l^–1 of cyanide. Because there was almost no incubation period required for the enzyme reaction, the method is very rapid and has potential for automation for the routine analysis of environmental and clinical samples. Ten-fold concentrations of acetate, ascorbate, citrate and iodide were found to interfere with the determination of cyanide concentrations <1 mg l^–1.