Changes in the plasma membrane phospholipid composition may alter the structure and/or fluidity and lead to a variety of changes in membrane functions. Dietary fats are known to influence the composition of lipids in the plasma membrane. The purpose of this investigation was to compare effects of dietary linoleic and stearic acid on the composition of the phospholipids in the plasma membranes of mammary tumors in A/St mice.Plasma membranes were isolated and lipids were extracted. Phospholipids were separated by thin-layer chromatography and identified by detection with molybdenum blue reagent. The Rf values and integration of optical densities were used to compare phospholipid composition in membranes of tumors from mice fed experimental diets. It appears that the amount of dietary fat, but not the type, affects the phospholipid distributions. The phosphatidylinositol was the phospholipid most affected, representing the smallest amount in membranes from tumors in mice fed the low fat diets. / Department of Biology