Age-associated structural changes in collagenous residues isolated from the loose connective-tissue network found within bovine biceps femoris muscle were studied with collagenase used as a structural probe. Rate of enzymatic digestion was measured by following the release of soluble protein, hydroxyproline, and ninhydrin-positive material into the medium surrounding the fibrous residues. Under the conditions of the experiment, 8–11% of the protein and 8–21% of the hydroxyproline in the residues were solubilized after 12 hr of incubation with collagenase. Samples from four age groups were studied: Group I, veal, 40–49 days old; Group II, steers, 403–495 days old; Group III, cows, 4 years, 8 months to 5 years, 5 months old; Group IV, aged cows, 10 years, 2 months and 10 years, 5 months old. The groups ranked I, III, IV, and II, from fastest to slowest, in rate of hydrolysis by collagenase. However, it was postulated that the larger amounts of lipid associated with the Group II samples acted to shield collagenase-labile bonds and cause a slow rate of hydrolysis. Making this assumption, it appears that susceptibility to collagenase digestion of loose connective tissue follows an age pattern similar to that reported for skin and tendon collagen.