Phorbol esters which activate protein kinase C (PKC) have been shown to enhance experimental lung metastasis. Therefore, it was reasoned that inhibitors of PKC might also modulate metastasis. We have investigated this possibility using a PKC inhibitor, MDL 27,032 4-propyl-5(4-pyridinyl)-2(3H)-oxazolone, as well as staurosporine and H-7. Treatment of B16F1 murine melanoma cells with MDL 27,032 for 24 h in culture and subsequent i.v. injection of the cells into mice resulted in greater than 90% inhibition of lung metastasis. Inhibition of metastasis was time dependent, with 90% of maximum inhibition occurring by 8 h of incubation. The 50% inhibitory concentration (IC50) for inhibition of metastasis with MDL 27,032 was 7 microM, a value similar to that for the inhibition of B16F1 membrane-associated PKC (IC50 = 13 microM) but not cytosolic PKC (IC50 = 54 microM). B16F1 cells treated with MDL 27,032 for 24 h were less adherent than untreated cells to extracellular matrix/basement membrane proteins. Adhesion to fibrinogen and collagen IV was inhibited (IC50 = 6 microM and 48 microM, respectively) by MDL 27,032, whereas adherence to laminin and fibronectin was not affected, indicating that the drug affects specific adhesion molecules. MDL 27,032-treated cells were also found to be less adherent than untreated cells to human umbilical vein endothelial cells. The phosphorylation of an 80-kDa B16F1 cell plasma membrane protein was stimulated under conditions known to stimulate PKC activity, and MDL 27,032 inhibited this phosphorylation in a dose-dependent manner. MDL 27,032 was more potent than H-7 for the inhibition of metastasis but was significantly less potent than staurosporine. These results support the hypothesis that there is a critical role for PKC-mediated phosphorylation of cell surface adhesion receptors in metastasis.