Although attention is usually focused upon cholesterol as the chief dietary sterol of human beings, one half or more of the total sterols consumed by man constitute many sterols other than cholesterol. These include sterols from plants and seafoods. The metabolism of only one plant sterol, Beta-sitosterol, has been relatively well studied. It and other plant sterols have been associated with a new sterol storage disease, "Beta-sitosterolemia and xanthomatosis". There is a great lack of knowledge about the metabolism of the numerous other sterols contained in shellfish and algae. These include brassicasterol, 24-methylene cholesterol, 22-dehydrocholesterol, a 26-sterol, and a 29-sterol. Seaweed contains an additional sterol of interest, fucosterol. In the proposed protocol, we pose the following questions: To what extent are these different sterols absorbed by the human intestinal tract? How do these sterols affect the absorption of cholesterol? Would they be hypocholesterolemic? Are these sterols or their derivatives found in bile, gallstones and atherosclerotic plaques? What is the origin of these different sterols, particularly in shellfish, biosynthetic or dietary from phytoplankton? Sterols will be purified from shellfish and algae for studies of their absorption and turnover in man. Animals such as the oyster or the clam will be labelled with acetate or mevalonate and cholesterol biosynthesis measured. Controlled dietary studies will be done by feeding phytoplankton in culture to clams and oysters incubated under controlled conditions. Subsequent feeding studies using shellfish sterols will be done in man with sterol balances being carried out. These studies will answer fundamental biological questions as well as having pertinent medical implications to certain diseases such as atherosclerosis and cholesterol gallstones.