The selective reaction of SH containing proteins and peptides with NTCB (2-nitro-5-thiocyanobenzoic acid) has been reported (Degani, Y., & Patchornick, A. (1974) Biochemistry 13, 1; Jacobson, G.A., Schaffer, M.H., Stark, G.R., & Vanaman, T.C. (1973) J. Biol. Chem. 248, 6583). With this reagent, cysteinyl peptide bonds are selectively cyanylated and subsequently cleaved under alkaline conditions. In the present study we have successfully cleaved the beta-chains of guinea pig hemoglobin at the single cysteine and the peptides thus obtained were separated. However, the C-terminal peptide was blocked at its N terminal by a thiazolidine ring and hence could not be used for Edman degradation sequence analysis. Deblocking of this peptide was successfully done by Raney nickel in the buffer medium of pH 7.0, and also in water, at 50 degrees C for 6 to 10 h. The Raney nickel reagent is used in large excess by weight (at least ten times the weight of sulfur compound) over the compound to be desulfurized. Under these conditions, control experiments on cysteine, methionine, and some other amino acids showed that only the sulfur containing amino acids are degraded by Ni(H). Cysteine and methionine were rapidly converted to alanine and beta-aminobutyric acid, respectively. Gel electrophoresis of the iminothiazolidine peptide after exposure to Ni(H) showed no breakage of the chain.