Amino acid sequences around the disulphide bridges of the heavy chain of an imnmunoglobulin of the y2 subclass have been studied. The protein was digested with pepsin and the digest fractionated by Sephadex. Screening of the eluate by one-dimensional electrophoresis of oxidized and unoxidized samples was used as an assayand pools offractionswere prepared. Identificationbydiagonal electrophoresis of several interand intra-chain disulphide bridgeswas done on the pooled fractions. The inter-heavy-chain bridged peptide included four cystine residues. Comparison with proteins of other human subclasses indicated that the intrachain bridges identified are the bridges of the invariable section of y2 heavy chains. The amino acid sequence of one cysteic acid peptide that may have been derived from the variablepart ofthe molecule was determined. Partial reduction followed by carboxymethylation with radioactive iodoacetate of two proteins of the y2 class showed a number of labelled peptides that could be identified as being related to the interchain bonded cystine residues.