In our search for the mechanisms by which the drug 1-O-alkyl-2-O-methylglycero-3-phosphocholine (AMG-PC) inhibits tumor growth and metastasis, we have detected a metabolite, 1-O-alkyl-2-O-methylglycerol (AMG), in membranes of MO4 mouse fibrosarcoma cells grown in the presence of the drug. Synthetic AMG inhibited the activation of highly purified human protein kinase C by diacylglycerol in the presence of phosphatidylserine. Furthermore, AMG also inhibited the receptor-specific binding of 3H-phorbol-12,13-dibutyrate to human HL-60 promyeloid leukemia cells in a dose-dependent fashion. AMG-PC was not effective or much less so in these assays. We suggest that interaction of the metabolite AMG with protein kinase C may inhibit stimulus-response coupling in tumor cells and may thus potentially contribute to the mechanism by which AMG-PC exerts its anticancer activities.