An acute experiment was conducted to determine the short-term effect of an LD50 dose of AFB1 on rats fed a diet containing either 30% corn oil (unsaturated) or 28% beef fat (saturated) for 3 weeks. The male weanling Wistar rats weighing 50-65 g were fed the respective dietary fats for 2 weeks and then given a single dose of AFB1 (7 mg/kg body weight) dissolved in dimethyl sulfoxide by gastric intubation. One week later they were sacrificed and assays for p-nitroanisole demethylase and benzpyrene hydroxylase were performed on liver microsomes to determine the activity associated with the two types of dietary fat. The rats fed corn oil or unsaturated fat had lower total liver fat and a lower mortality rate than those fed beef fat. The basal levels of liver microsomal oxidase activity were higher in rats fed the corn oil diet than in those given the beef fat diet.