The serum levels of apo B and apo E in rats fed on a diet rich in cholesterol before and after thymus treatment were determined by the authors. The diet enriched with cholesterol increases the serum levels of apo E and of the large and small species of apo B. After treatment the large apo B and the small one strongly decrease, while apo E increases further. These data support the hypothesis that the dropping of total cholesterol, after thymus treatment, cannot be ascribed to apo E decrease but possibly to B-apoproteins.