Diabetes prone NOD female mice were fed diets containing different proteins from just before weaning. Only mice receiving meat meal or casein as the protein source developed diabetes at the rate expected from this colony. Lactalbumin and gluten did not precipitate diabetes except in a small number. Casein hydrolysate in lieu of protein protects against overt diabetes, but only if introduced early. The animals which did not show overt diabetes nevertheless had intermittent trace glycosuria and the majority showed mild degrees of periinsular lymphocytic infiltration.