Human A431 epidermoid carcinoma cells in culture exhibit epidermal growth factor (EGF)-induced "down-regulation" of cell-surface and total cellular (Triton X-100 extractable) EGF receptors caused entirely by an enhanced rate (4-fold) of receptor inactivation [Krupp, M. N., Connolly, D. T. & Lane, M. D. (1982) J. Biol. Chem. 257, 11489-11496]. The following observations show that this enhanced rate of EGF receptor inactivation is closely correlated with an increased cellular activity of plasminogen activator (PA), a serine protease. First, EGF-induced down-regulation of cell-surface and total cellular EGF receptors and the concomitant increase in cellular PA activity occur with identical kinetics, the t 1/2 for both processes being 3-3.5 hr. Second, the EGF dose-response curves for down-regulation of total cellular EGF receptor and increased PA activity are similar. The EGF concentrations for half-maximal responses of both processes are 10-15 nM and 20 nM, respectively. Third, the removal of EGF from previously down-regulated cells results in the recovery of total cellular EGF binding activity with a concurrent loss of cellular PA activity. Fourth, blocking PA synthesis or activity with cycloheximide or dexamethasone prevents down-regulation of the EGF receptor. Fifth, the addition of leupeptin, an inhibitor of PA and plasmin action, blocks EGF-induced receptor down-regulation as well as the increase of PA activity. That EGF receptor down-regulation is independent of plasminogen per se in the culture medium suggests that PA-mediated events may initiate the rapid inactivation of the EGF receptor that occurs during down-regulation.