The present paper describes the preparation of benzyloxycarbonyl-2-bromoethylamine, benzyloxycarbonyl-2-iodoethylamine, S-2-benzyloxycarbonylaminoethyl-L-cysteine, S-2-aminoethyl-L-cysteine, and polymers of the last two compounds. The preparation of carboxymethyl 2-aminoethyl sulphide is also described and it is reported that the latter compound does not act as an inhibitor for the hydrolysis of toluene-p-sulphonyl-L-arginine methyl ester by trypsin.