The Tetronic series of polymeric surface-active agents were screened for hypocholesterolaemic activity in rats fed on a semi-synthetic hypercholesterolaemic diet. Only Tetronics 701 and 702 were active and the former was further investigated. Tetronic 701 lowered serum and liver cholesterol in rats fed on a semi-synthetic diet, with or without cholesterol, but not in rats fed on stock laboratory diet. A dose-related growth depression was observed. The compound was hypocholesterolaemic in chicks and rabbits fed on cholesterol-containing diets. The uptake of a single dose of cholesterol into liver and serum was inhibited in rats given Tetronic 701. Tetronics 701 and 702 were effective in precipitating cholesterol from mixed micelles in vitro. Non-hypocholesterolaemic Tetronics were inactive in this respect. A series of tetraesters of tetronic 701 were prepared and tested in rats fed on a semi-synthetic hypercholesterolaemic diet. Several were hypocholesterolaemic and the tetrabenzoate was of especial interest in that it depressed growth less than did Tetronic 701 itself.