The role of sulphhydryls in the chelation and reduction of iron by digests of chicken muscle was investigated. The salt-soluble (DSSP) and myofibrillar (DSIP) proteins were extracted from chicken muscle and digested with pepsin/pancreatin, and the digests were then incubated with ferric iron. The DSIP digest bound twice as much iron as the DSSP digest; about half the bound iron was ferrous. Iron binding was accompanied by a 74% drop in free sulphhydryls in the DSSP digest and a 72% drop in the DSIP digest. Modification of sulphhydryls in the digests by either oxygen, N-ethylmaleimide or p-chloromercuribenzoate led to a loss of ∼90% in reactive sulphhydryl groups and was accompanied by losses in iron binding which averaged 79% for DSSP and 67% for DSIP. Losses in ferrous bound iron were proportional to the loss in total bound iron. It was concluded that for the muscle protein fractions, sulphhydryl residues are the principal component responsible for iron binding, but that other residues may play a minor role.