Both epidemiological studies in humans and laboratory studies in animals have provided evidence that high fat diets enhance mammary tumor growth. These data indicate that this effect of lipids on breast cancer is due to an enhancement of cell growth during this "promotional" phase of tumor development, and that a proper hormonal enviornment is necessary for its maximal expression. The biochemical processes responsible for this lipid-mediated enhancement of tumor growth are poorly understood, but one promising possibility is that certain alterations in the lipid composition of tumor cell membranes induced by qualitative and quantitative differences in dietary lipid may significantly amplify hormonally mediated stimuli. Recent evidence suggests that these effects on hormone activity may be due to either lipid induced changes in the physical chemical properties of the membranes themselves or secondary to membrane mediated effects on intracellular prostaglandin metabolism. It is the objective of this study to examine these possible mechanisms by characterizing the membrane lipids of mammary tumors from animals maintained on various lipid diets and by correlating these findings with measurements of hormone receptor activity and tissue prostaglandin levels. Furthermore, this study will examine whether these lipid effects can be modified by specific hormone excesses or deficiencies. It is hoped that the information obtained from these studies ultimately will extend our knowledge of tumor promotion far enough to enable therapeutic programs to be developed that would be capable of inhibiting, or at least delaying, breast cancer.