The effectiveness of the two proteolytic enzymes, pepsin and proctase (isolated from Aspergillus niger), was investigated in order to compare their relative efficiencies for the solubilization of collagen from bovine skin. Some of the molecular properties of gelatins derived from these collagen preparations were also studied. Pepsin and proctase solubilized collagen with similar yields (75% and 76% of total collagen as calculated from hydroxyproline). The relative amounts of collagen extracts converted to gelatin were also comparable (32% and 39%, respectively). However, proctase-extracted collagen exhibited a decrease of high-molecular-weight components compared to pepsin-prepared collagen. Furthermore, gelatin obtained from proctase-extracted collagen showed significant proportions of molecular species smaller than collagen alpha-chains. Type III to type I collagen ratios were also analyzed in both preparations. The results indicate an increase of the proportion of type III collagen in proctase extract as compared to pepsin extract. Rheological properties of gelatin obtained by proctase solubilization exhibited a very significant decrease of gel strength, viscosity, and turbidity parameters as compared to pepsin-derived gelatin. These parallel biochemical and biophysical data indicate that proctase-prepared gelatin is markedly altered compared with pepsin-derived gelatin.