Primary cultures of human endometrial stromal cells expressed a single class of specific high-affinity binding sites for urokinase plasminogen activator (UPA) with a dissociation constant KD 1.0 nmol/l and saturation at 2.0 nmol/l. Similar binding data and number of free binding sites, about 200 fmol/mg protein, were found for UPA in complex with its inhibitor plasminogen activator inhibitor-1 (PAI-1). These binding data agree with those reported for the specific cell surface receptor for UPA, and stromal cell expression of UPA receptor mRNA was identified in Northern blots. Cell surface-bound UPA was degraded at 37 degrees C. Degradation of complexed UPA was more efficient than that of free UPA. Degradation of free UPA did not require prior binding to endogenous PAI-1. Degradation of both free and complexed UPA was reduced by 70% by colchicine, chloroquine and methylamine, indicating that degradation involved both internalization and lysosomal enzymes. Furthermore, degradation was independently inhibited by about 70% with anti-UPA receptor antibodies and receptor-associated protein, indicating that the UPA receptor as well as one or more receptors of the low-density lipoprotein (LDL) receptor supergene family were involved in the degradation process. Receptor-associated protein ligand blotting demonstrated a major band co-migrating with the LDL receptor-related protein or glycoprotein 330/megalin, and a minor band co-migrating with the very low-density lipoprotein receptor. Immunoblotting positively demonstrated expression of LDL receptor-related protein, but not glycoprotein 330.