The fatty acid composition of plasma membrane fractions of mammary adenocarcinomas from Strong A female mice fed diets varying in the amounts of linoleic and stearic acid were studied. Studies have indicated that linoleic acid is a tumor promoter and stearic acid is a tumor inhibitor. Studies have also shown that the plasma membrane plays an important role in the cell's responses to growth factors and in the development of the tumor.The effects of varying amounts of dietary 18:0 and 18:2 on the plasma membrane were studied. The purpose of the project was to determine the effects of these dietary fatty acids on the concentration of fatty acids in the plasma membrane.The Strong A mice were fed a high fat (15%) or low fat (5%) diet. The high fat diets were: SA-1 (13.2% 18:0, 0.8% 18:2), SA4 (10.3% 18:0, 3.0% 18:2), and SAF (0.4% 18:0, 11.4% 18:2).Thelow fat diets were: STOCK (0.3% 18:0, 1.5% 18:2) and CO (0.1% 18:0, 3.0% 18:2).A decrease in linoleic acid content was noted in all membranes except for the large tumors from mice fed the SAF diet which contained 3.5% as compared to 2.7% linoleic acid in stock tumors. There was also a very large increase in the amount of 18:0 in the SA-4 large (24.1%) and small (16.9%) as compared to 8.1% for the stock tumors. In the rest of the dietary groups there was not a significant difference in the amount of 18:0 as compared to the stock, although there was a small increase in each diet.