Rabbits were weaned to different semipurified diets to determine which characteristics of the diet were required to maintain the hypercholesterolemia of the suckling period. Gradual weaning to a diet similar in fat, cholesterol, protein and carbohydrate content to rabbit milk maintained and enhanced the hypercholesterolemia. Reduction of the cholesterol content, and to some extent the fat content of this diet, decreased the plasma cholesterol level, but the type of dietary protein did not alter the results. After weaning, however, animals fed milk protein (casein) in the absence of dietary cholesterol became hypercholesterolemic, whereas those fed plant protein (soy protein isolate) remained normocholesterolemic.