A study of the influence of chemical modifications on the activity of Achromobacter iophagus collagenase (EC 3.4.24.8) has led to the following conclusions: a modification of 4 out of 80 COOH groups with carbodiimide led to 90% loss of enzymic activity. A 70% inactivation was found after modification of two tyrosines out of 30 with tetranitromethane. The modification of four to six tryptophans out of 16 with 2-hydroxy-5-nitrobenzyl bromide decreased enzyme activity to 36%. This inactivation is accelerated in the presence of collagen. An increase of reagent/enzyme molar ratio led to a modification of 16 tryptophan residues and denaturation of Acahromobacter collagenase. A modification of two arginines out of 18 with 1,2-cyclohexanedione and eight NH2 groups out of 24 with 2,3-dimethyl maleic anhydride does not change the collagenolytic activity. All NH2 groups become available for 2,3-dimethyl maleic anhydride after dissociation of the dimer. A possible analogy of hydrolytic site of collagenase with that of two other known bacterial metalloproteinases (thermolysin and Bacillus subtilis neutral proteinase (EC 3.4.24.4)) is discussed.