5 alpha-Cholest-8(14)-en-3 beta-ol-15-one, a hypocholesterolemic agent and a potent inhibitor of sterol biosynthesis, inhibited the growth of CHO-K1 cells incubated in medium containing fetal calf serum. The concentration of the oxysterol required to inhibit growth by 50% was 13 microM. Sodium oleate (82 microM) reduced the inhibitory effects of the sterol, and increased the concentration of the 15-ketosterol required to cause a 50% inhibition of growth to 25 microM. The ACAT inhibitor N'-(2,4-difluoro-phenyl)-N-[4-(2,2-dimethylpropy)-phenyl]-methyl)- N- heptylurea (5 microM) abolished the effect of sodium oleate, and reduced the concentration of the 15-ketosterol required to inhibit growth by 50% to 5 microM.