The present paper is concerned with the heredity of the speciesspecific properties in animals as shown by serum reactions. The problem has been set forth before by von Dungern and Hirschfeld (1) and J. Loeb (2), but no experiments have been carried out by these authors with the exception of an attempt to examine the hemoglobin crystals of the mule.2 The material used in our work was the blood of the most accessible animal species-hybrid—the mule—and comparison has been made with the blood of the horse and the donkey. For this purpose it was necessary to have at our disposal a method which would allow of sharp distinction between the species-specific substances of so closely related animals. The differentiation of the serum proteins of these animals by means of the precipitin and complement-fixation tests is possible, according to our experience, with even greater facility when partially absorbed immune sera are employed. But the differences so determined are not very considerable. For this reason we tried first to differentiate the specific substances contained in the red blood cells with immune agglutinins. In this way satisfactory results were easily obtained.