Rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1.) was purified from chicken livers and its amino acid sequence was determined. The enzyme has a specific activity of 676 IU and a molecular weight of 32,255. The primary structure of 289 amino acids was solved by sequential Edman degradation of overlapping peptides obtained by selected enzymatic and chemical cleavages. The amino terminus was blocked, and the carboxy-terminus was heterogeneous. Comparison of the primary structure with bovine liver rhodanese showed 212 identically matched amino acids, and the majority of amino acid differences were conservative substitutions. Reaction of the enzyme with a 1.4-fold molar excess of [2-14C]iodoacetate led to inactivation of the enzyme and carboxymethylation of Cys-244; this modification was blocked by the substrate thiosulfate.