The effect of dietary cholesterol and the type of protein in the diet on the cholesterol concentration and the lipoprotein pattern in serum was studied in male chickens. The chickens were divided into six groups each of five animals, each group receiving a different diet for 6 weeks. Two groups received a commercial diet, to one of which cholesterol (1% wt/wt) had been added. The remaining four groups received a semipurified diet containing either casein or soybean protein as protein source as follows: casein, casein plus cholesterol (1% wt/wt), casein plus arginine (0.85% wt/wt) and soybean protein. No change in the concentration of cholesterol in serum was observed in the chickens fed diets without added cholesterol. When cholesterol was added to the diets, there was a significant increase in the cholesterol concentration. After the animals were fed the diets for 6 weeks, blood was taken for the examination of serum lipoproteins. In the experimental groups on the cholesterol-containing diets, a shift in the lipoprotein pattern from the low-density lipoproteins to the intermediate-density lipoproteins and the very low density lipoproteins was seen. This was confirmed by determination of the composition of the lipoprotein particles and by their appearance under the electron microscope. However, no differences in the density profile and lipoprotein composition could be observed between the groups receiving a cholesterol-free diet containing casein and soybean protein.