Plasmin was found to degrade the fibronectin (Fn) mesh produced by cultures of normal rabbit corneal fibroblasts, cause breakdown of F-actin-containing microfilament bundles ("stress fibers"), and increase levels of active type I interstitial collagenase (MMP-1) in the medium. Fibroblast cultures derived from alkali-burned, ulcerating rabbit corneas also responded to plasmin by secreting collagenase, detected only in active form. Moreover, harvests from organ cultures of ulcerating corneas not only had higher levels of urokinase-like plasminogen activator (uPA) than normal cultures but also had higher levels of Fn degradation fragments. The results are consistent with reports that indicate that perturbation of the alpha 5 beta 1 integrin (Fn) receptor by proteolytic fragments of Fn causes the increased synthesis and secretion of MMP-1. The uPA/plasmin system, therefore, might have an important role in regulating collagenase synthesis, secretion, and activation during wound remodelling and stromal ulceration.