1. Two groups, each of six rabbits, were fed on semi-purified diets containing either 400 g casein or 400 g soya-bean protein/kg for 20 d and then the diets of the two groups were crossed-over.
2. Just before the cross-over, the serum cholesterol concentration (mean ± SE) was 3068 ± 592 and 800 ± 143 mg/l for the groups fed on casein and soya-bean protein respectively.
3. Changes in the serum cholesterol concentration were observed 1 d after crossing-over the diets. By 10 d, the cholesterol levels in the two groups had also crossed-over.
4. The changes in serum cholesterol level after the cross-over were reflected in the very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL).
5. Lipoprotein protein concentrations in the LDL changed in the same way as cholesterol. In the VLDL however, the protein concentration decreased in both groups after the change in diet.
6. The cholesterol:protein values for the LDL and VLDL markedly increased in the rabbits changed from the soya-bean-protein diet to the casein diet, reaching a maximum 2 d after the cross-over. In the animals switched from casein to soya-bean protein, the values progressively declined.
7. The source of dietary protein exerts a rapid effect on the composition of both the VLDL and LDL which is proposed to be attributed to changes in the number and size of lipoprotein particles.