N-acylated mercapto-.alpha.-amino acids of the general formula ##STR1## are prepared by reacting a nitrile of the general formulaR.sub.1 -CN (II)at a temperature between 0.degree. C. and 100.degree. C. in water or in a mixture of water and an organic solvent which is at least partially miscible with water, in the presence of a base with a mercapto-.alpha.-amino acid of the general formula ##STR2## and subsequently releasing the N-acylated mercapto-.alpha.-amino acid of general formula (I) with an acid. Mercapto-.alpha.-amino acids can be selectively acylated on the amino group in this manner in a relatively simple manner.