activity after 12h; however, on being left for a further 24-36h, traces of collagenase activity were observed surrounding the cells. It would appear that this inhibitor diffused away from the cells, and, as a consequence, the cells slowly regained trypsinlike activity. When Tos-Lys-CH,C1 was included in the collagen gel, the inhibition of collagenase activity was maintained for 48 h. With cells treated with 4-methyl-umbelliferyl 4-guanidinobenzoate hydrochloride, no collagenase activity was observed over 48 h, indicating irreversible inhibition of the cell-surface enzyme required to activate the zymogen of collagenase. This latter result would be expected from an active-site titrant that forms an irreversible complex with the enzyme (Chase & Shaw, 1967). It was further demonstrated that, with collagen gels containing Tos-Lys-CH,CI plus added chymotrypsin, cells pretreated with Tos-Lys-CH,CI exhibited collagenase activity after 12h. This result, taken with the results described above, is clear proof that the cells export a zymogen of collagenase requiring proteolysis by chymotrypsin or the cell-surface trypsin-like enzyme before activation and the demonstration of collagenolysis. Since none of the inhibitors described above has any effect on collagenase itself, the effect of these agents must be on the activation process. We believe that the cell-surface enzyme, which is not inhibited by high-molecular-weight inhibitors of trypsin, has been shown to be capable of carrying out this activation. The presence of a,-macroglobulin and other extracellular inhibitors of collagenase in in vivo would prevent collagenolysis taking place distant from cells exporting collagenase or its zymogen. The role of the cell-surface trypsin-like enzyme (which is not affected by serum inhibitors) in zymogen activation would explain the pericellular degradation of collagen fibrils observed in connective-tissue damage.