Eight human gingival fibroblast cell lines were cultivated on bovine collagen fibrillar mats thinly coated on over slips of tissue culture chambers. The responses of the cells to the collagen were studied by light and electron microscopy. It was shown that the collagenolysis was regional, occurring only in the immediate areas of the cells and resulted from the phagocytosis of collagen fibrils by the fibroblasts. There was no evidence by electron microscopy of an enzymatic lysis of the fibrils in the extracellular spaces. The fibrils were first seen within folds of the fibroblasts whose membranes contained multiple profiles of invaginating pinocytotic and/or membrane‐coated vesicles. The fibrils were then interiorized in slender cell processes and ultimately became enclosed within developing acid phsphatase positive lysosome‐like bodies. The process of phagocytosis and the sequences of development of the lysosomal enclosures are combined with the concept of “hydrolase secretion and recapture‐mediated endocytosis by fibroblasts” to derive a hypothesis for fibroblastic degradatin o