The purpose of this study was to examine the effects of various dietary fats on low density lipoprotein (LDL) binding in an in vitro system where receptor number is not regulated. Cynomolgus monkeys were fed diets containing 37% of energy from fat, with various degrees of saturation, and 0.4 mg/kcal cholesterol or low-fat (13% of energy), low cholesterol (0.03 mg/kcal) chow. Plasma LDL was isolated after 16 weeks. The fatty acid composition of LDL showed enrichment corresponding to the dietary fats consumed, and the high fat, high cholesterol diets produced marked hypercholesterolemia compared to chow feeding. Of those fed the high fat diets, monkeys fed the fish oil diet had the highest LDL cholesterol concentrations, 13.25 +/- 0.77 mmol/l, while those fed the safflower oil diet had the lowest, 7.51 +/- 3.31. LDL from chow fed monkeys had the lowest binding affinity; the Kd was 26.2 +/- 8.7 microg/ml, nearly twice that of the high fat diets (P = 0.003). No significant differences in binding were found between the different high fat diets, although there was a trend toward lower affinity in the diets enriched in polyunsaturated fat. LDL size was affected by diet with chow fed monkeys having the smallest average LDL, 259.3 +/- 1.7 A compared to the other groups (P = 0.03). Monkeys fed the fish oil diet tended to have smaller LDL, but this was not significantly different from the other high fat diets. Binding affinity was correlated with LDL size, r = 0.54, P < 0.01. LDL composition, as measured by apo B/cholesterol ratio, was altered by feeding a high fat, high cholesterol diet. The ratio was reduced in the LDL samples from monkeys fed the high fat diets compared to those fed chow, but this ratio was not significantly correlated with binding. Thus, it appears that increasing dietary fat and cholesterol intake increases LDL size and binding affinity, such that LDL metabolism may be altered independently from effects on receptor number; the type of dietary fat does not seem to influence this process when fat and cholesterol content is very high.