[en] The D-alanyl-D-alanine carboxypeptidase excreted by the penicillin-sensitive Streptomyces strain R 61 is an anionic protein at pH 6.5. It does not require cations for its action. By comparison with the penicillin-resistant DD-carboxypeptidase from Streptomyces albus G, the penicillin-sensitive R 61 enzyme has high Km values with all the peptides which were used but the alignment of catalytic groups is more effective in promoting hydrolysis of the peptides (high Vmax values). Moreover, the substrate structures required by the two enzymes show some differences. In contrast to the albus G DD -carboxypeptidase which is either not, or very little, affected by penicillins, the R 61 enzyme is inhibited by low dose levels of penicillin G and by cephalothin