The literature on collagen has been reviewed, with especial emphasis on structural aspects of collagen and the collagenolytic effects of enzymes. The proteolytic enzyme of Streptotmyces griseus (Pronase) has been examined and shown to consist of four fractions, each having proteolytic activity. The collagenolytic effect of the above enzyme and its fractions, and to a lesser extent of other enzymes has been examined by a variety of methods, both physical and chemicals using both soluble and insoluble collagen as substrates. Pronase has been shown to have a collagenolytic effect on collagen in both the soluble and insoluble states, and the products of collagenolysis have been examined. The action of Pronase has been shown to result in peptide bond cleavage only in the terminal regions of the collagen molecule, resulting in the liberation of small peptides and free amino acids, while leaving the body of the molecule intact. Fractionation of Pronase-treated collagen after denaturation showed that the three constituent chains of the collagen molecule had been separated, indicating that the enzyme is also affecting lateral cross linkages. An extract of sisal has been shown to have a proteolytic effect using gelatine as substrate, but a very slight collagenolytic effect, which is limited to collagen in solution. Insoluble collagen has been shown to contain a small quantity of non-protein-nitrogen material which can be removed by physical methods, and which consists of free amino acids, small peptides and a carbohydrate component.