Pure collagenase (clostridiopeptidase A, EC 3.4.24.3) having a molecular weight of 70 000 was obtained from the culture medium of Clostridium histolyticym by a combination of ultrafiltrations, molecular sieve, affinity and hydrophobic chromatography. The value of its specific activity is the highest of those described previously but 6-times lower than that of the collagenase from Achromobacter iophagus (EC 3.4.24.8). Its amino acid composition differs from previous data, namely by the presence of cysteine, methionine, tryptophan and O-phosphoserine residues. In contrast to Achromobacter collagenase it does not dissociate in subunits during the deactivation by EDTA or LiCl/glycine buffer at pH 10.5. Existence of multiple forms of Clostridium collagenase previously described is discussed as being due to autolysis of a single molecular species or to a different degree of phosphorylation.