The relevance of urokinase receptors to urokinase-mediated laminin degradation was investigated in cultured colon cancer. Six colon cancer cell lines degraded laminin in a plasminogen-dependent manner. The ability of the individual cell lines to cleave the glycoprotein correlated well (r2 = 0.9242) with the amount of urokinase recovered from the cell surface by a mild acid treatment. A radioreceptor assay indicated that colon cancer cells most active in degrading the laminin, possessed the largest number of urokinase receptors. Moreover, acid treatment which depletes the receptors of endogenous plasminogen activator augmented the specific binding of radioactive urokinase to the colon cancer cells by 12-200%. A cell line (HCT 116) which displayed 1.1 x 10(5) receptors/cell the majority of which were occupied with endogenous urokinase was selected and the effects of a urokinase receptor antagonist on laminin degradation determined. The peptide antagonist reduced laminin turnover by 60-80%. Morphological observations were consistent with these findings. Plasminogen-treated HCT 116 cells retracted from the culture dish and many cells were observed in the culture medium. This effect could be largely reversed by simultaneous treatment with the peptide antagonist. A poor correlation was found between laminin degradation and soluble urokinase (r2 = 0.1074). These data strongly argue for a role of the urokinase receptor in facilitating the action of the plasminogen activator in colon cancer.