[Evaluation of substrates for measuring cystyl-amino peptidase activity in various sera and tissues with respect to L-cystine-di-beta-naphthylamide, L-cystine-bis-p-nitroanilide, L-cysteine-beta-naphthylamide, and S-benzyl-L-cysteine-p-nitroanilide (author's transl)].