Summary Sixteen mucilaginous polysac-charides fed to chickens for 28 days at various levels between 1.0% and 0.5% of a casein-sucrose basal diet supplemented with 3% cholesterol exhibited hypocholesterolemic activity. Relative decreasing order of activity of 14 of these agents tested at the 3% dietary level is: carrageenin, salep root, guar gum, karaya gum, locust bean gum, ghatti gum, psyllium seed, tragacanth gum, shiraz gum, dextran CR, pectin N. F., agar, alginic acid and dialdehyde gum XO-200. The two other active agents, fucoidin and polysaccharide Y-1401, were tested only at 2%) of the diet. Carrageenin demonstrated activity at the 1%. dietary level and guar gum as low as 0.5%. Carrageenin was active in a soybean protein-glucose synthetic diet and in a commercial-type diet supplemented with 3% cholesterol. Significant reductions in plasma cholesterol levels were obtained with 3% and 2% guar gum in the casein-sucrose basal diet unsupplemented with cholesterol, indicating that guar gum can lower the endogenous level of chickens consuming this experimental diet; 3% carrageenin was inactive under these conditions. Minimal reductions (< 9%) in body weight gain were obtained with 3% carrageenin, guar gum and pectin N. F. in the casein-sucrose basal diet supplemented with 3% cholesterol; at the 2% dietary level, pectin N. F. still inhibited body weight gain, whereas carrageenin and guar gum gave a slight increase. No apparent reduction in food consumption was observed with carrageenin, guar gum and pectin N. F. at the 3% dietary level. A relationship between polysaccharide structure and hypocholesterolemic activity was not established, although there was a possible correlation between the activity of certain preparations of a particular polysaccharide and parameters such as water solubility, viscosity and molecular weight.