The silkworm larvae reared on artificial diet were more susceptible to the infection per os with nuclear polyhedrosis virus than those reared on natural diet and furthermore the anti-viral activity in the digestive juice of the former was weaker than that of the latter. The fluorescent colours of the digestive juices from the larvae fed on natural or artificial diets, were different from each other, being red in the former and yellow in the latter. A protein having red fluorescence was fractionated from the digestive juice of silkworm larvae fed on natural diet, mulberry leaves. The fluorescent protein fraction inactivated the nuclear polyhedrosis virus and the inactivating potency was almost same both in the digestive juice and in the buffered solution containing this fluorescent protein fraction at nearly equal concentration of that in the digestive juice itself. It is concluded that this red fluorescent protein fraction may play an important role in the protective mechanism in silkworm larvae for per os infection by the silkworm nuclear polyhedrosis virus and that the fraction is not observed in the digestive juice from the larvae reared on artificial diet.