THE germ-free animal with its more controlled environment and reduced reticuloendothelial system has become a valuable tool in microbiological and immunological investigations. In the germ-free rat a deficit in immune proteins causes low levels of serum β- and γ-globulins1. However, the animal is still subject to the variable reticuloendothelial system stimulating factors occurring in the steam-sterilized diets generally used. This communication presents data on serum proteins of germ-free rats fed a chemically defined, water-soluble `antigen free' diet which was filter-sterilized. This type of diet, composed almost entirely of low-molecular-weight compounds and free of material of bacterial origin, should further reduce the level of reticuloendothelial system activity because of the absence of stimulation caused by high-molecular-weight substances and by killed and disintegrated microbial agents.