It is well documented that cyclopropenoid fatty acids included in the diet with various mycotoxins or other carcinogens greatly enhance the induction of liver cell cancer in rainbow trout. These fatty acids occur in a variety of seed oils consumed by man and animals. The mechanism for this cocarcinogenic action remains unexplained. It may be due to their adverse effects on lipid metabolism thereby affecting the normal functioning of cellular membranes or to changes in cellular lipid composition which alters the activity of hepatic drug-metabolizing enzymes. In the proposed project rainbow trout will be reared on diets containing various levels of cyclopropenoid fatty acids. Plasma and cytoplasmic liver cell membranes will be isolated and characterized chemically and by electron microscopy. Isolated membrane fractions will be studied to determine the relationship between cyclopropenoid fatty acid induced lipid alterations and; (1) the structure of membrane lipids, (2) the functioning of membrane-bound transport enzymes, (3) the topography of the plasma membrane, and (4) the functioning of hepatic drug-metabolizing enzymes. These studies will provide information on the mechanism by which dietary cyclopropenoid fatty acids as cocarcinogens affect the functioning of cellular membranes. Moreover, these data should help to assess the possibility of using this system as a means of studying the relationship between altered membrane lipids and cocarcinogenicity.