A sensitive quantitative method has been developed to determine the number of disulfide bonds in peptides and proteins. The disulfide bonds of several peptides and proteins were cleaved quantitatively by excess sodium sulfite at pH 9.5 and room temperature. Guanidine thiocyanate (2 M) was added to the protein solutions in order to denature them and thereby make the disulfide bonds accessible. The reaction with sulfite leads to a thiosulfonate and a free sulfhydryl group; the concentration of the latter was determined by reaction with disodium 2-nitro-5-thiosulfobenzoate (NTSB) in the presence of excess sodium sulfite. The synthesis, purification, and characterization of NTSB are described. The assay is rapid, requiring 3-5 min for oligopeptides and 20 min for proteins, and is as sensitive and quantitative as the sulfhydryl group assay employing 5,5'-dithiobis(2-nitrobenzoic acid) (Ellman's reagent). It can be used for the analysis of as little as 10(8) mol of disulfide bonds, with an error of +/- 3%.