The effects of a potent hypocholesteremic agent, chlorophenoxyisobutyrate (CPIB or clofibrate) on the growth, lipid content, and incorporation of 14C-labeled precursors (sodium [1-14C]acetate and [l-14C]palmitate) into various lipid fractions of Tetrahymena pyriformis, strain E, were investigated. Addition of CPIB induced a striking decrease in incorporation of C-labeled precursors into phospholipids. Synthesis from 14C-acetate of a sterol-like lipid, tetrahymanol, which is the most abundant neutral lipid and exclusively rich in the surface membranes, was also markedly inhibited by CPIB. Experiments with uC-acetate showed increased incorporation of 14C-radioactivity into triglyceride associated with a corresponding decrease in phospholipid biosynthesis.