A simple method for introducing, in buffered saline, a reactive sulfhydryl group on water-soluble molecules bearing an alkyl-amino group is described. This method is based on the use of two water-soluble reagents: 2-iminothiolane and 6,6'-dithiodinicotinic acid. The first one is open upon reaction with an amino group, and the generated thiol group is immediately protected by action of the second reagent. The optimal conditions were determined by taking into account the stability and the reactivity of both reagents with regards to pH and temperature. This method was validated through two applications, the substitution of bovine serum albumin with a bromoacetyl peptide and the substitution of an amino link at the 5' end of an oligonucleotide by reaction with either a fluorescent tag, iodoacetamidofluorescein, or a bromoacetyl peptide, upon reduction of the protected disulfide bridge with a third water-soluble reagent, namely tris(2-carboxyethyl)phosphine.