The effects of amount and type of dietary fish proteins on plasma and liver cholesterol concentrations were evaluated in female rats. The isonitrogenous diets used contained 10 g cholesterol/kg and were carefully balanced for residual fat, cholesterol, Ca, Mg and P in the protein preparations. Cod meal, soya-bean protein or casein was incorporated into the diets as the only source of dietary protein at three levels: either 24, 48 or 72 g N/kg diet. Extra protein was added to the diet at the expense of the glucose component. In a second experiment soya-bean protein, casein, cod meal, whiting meal or plaice meal was added to the diet at a level of 24 g N/kg. When compared with casein, cod meal and soya-bean protein decreased plasma and liver cholesterol concentrations. A further cholesterol-lowering effect was achieved by increasing the proportion of either soya-bean protein or cod meal in the diet. Substitution of casein for glucose did not influence plasma and liver cholesterol concentrations. Plaice meal in the diet produced lower group mean plasma cholesterol concentrations than did whiting meal. In rats fed on the diet containing plaice meal, liver cholesterol concentrations were significantly lower than those in their counterparts fed on either cod meal or whiting meal. The present study demonstrates that different fish proteins in the diet have different effects on cholesterol metabolism and that the cholesterol-influencing properties of cod meal can be enhanced by the incorporation of higher proportions of this protein in the diet.