Healthy, nonobese, young rabbits developed hypercholesterolemia following daily intake of fresh egg yolk for 8 weeks. On a quantitative basis, this diet had a profound effect on serum cholesterol level which rose to 15 or 30 times the baseline value depending on whether the test group consumed the yolk content of one or two eggs during the study. Similar drastic changes observed in low density lipoprotein cholesterol levels in these test animals make this a useful method for defining the early pathological changes of atherosclerosis in man. In addition, the present study provides direct evidence of the deleterious effect of hen's egg yolk on the serum cholesterol level in an appropriate and easily reproducible animal model.