There is now ample evidence that the proteolytic action of urokinase (UK) is potentiated by a specific cell surface receptor. The present study was undertaken to assess the role of UK as a modulator of its receptor. GEO colonic cells, which secrete relatively low levels of UK (congruent to 0.1 nM/72 h per 10(6) cells) and display approximately 10(4) receptors per cell, 10% of which are "tagged" with the endogenous plasminogen activator (PA), was selected for the study. A 90% reduction in the specific binding of radioactive DFP-UK was observed for cells cultivated in the presence of two-chain (TC) UK (Mr 55,000). This only partly reflected occupation of the receptors with UK supplied in the culture medium, since the specific binding of the radioligand was still reduced by 60% after an acid pretreatment, which dissociates receptor-bound UK. The reduction in radioactive DFP-UK binding to cells treated with high molecular weight UK, either in the single or two-chain form, was both concentration and time dependent. Maximum reductions (70%) were achieved by treatment of the cells for 24 h with 1 nM of the plasminogen activator. In contrast, low molecular weight UK, which lacks part of the UK A chain, had no effect on ligand binding. Attenuation of radioactive DFP-UK binding to UK treated GEO cells was a consequence of a 60% reduction in the number of binding sites. Treatment of GEO cells with an antibody, which blocks the binding of endogenous UK to its receptor, augmented radioactive DFP-UK binding by two-fold. These data indicate that for one colonic cell line, at least, UK down-regulates its own binding site subsequent to it being bound to the receptor.