Excellent anticancer activity of 1,3-oxazol-4-ylphosphonium salts has been demonstrated in a total panel of NCI tumor cell lines. The average values of the antitumoractivity parameters of the most potent derivatives ranged from 0.3–1.1 μM (GI50), 1.2–2.5 μM (TGI), and 5–6 μM (LC50). The cytotoxicity vector of the compound 9 showed a high positive correlation with standard antitumor agents that induce