Bartonella muris appeared in the blood of all white rats, wild rats, Chinese hamster, and mice, from which the spleen was removed, but did not appear in that of splenectomized monkeys, rabbits, or guinea pigs. It has not been possible to transmit B. muris to normal rats, monkeys, rabbits, or guinea pigs, by intraperitoneal, intradermal, or intravenous injection of blood containing B. muris from splenectomized rats. In two instances an acute orchitis was induced in normal rats by injection directly into the testicle of blood or saline suspensions of the liver of splenectomized rats. The intracorpuscular elements occasionally identified as B. muris or as having appeared as a result of the inoculation. The acute orchitis of rats was transferable to normal rats in series. From the testicular tissue, as well as directly from the blood of a splenectomized rat, there was isolated in pure culture a microorganism which induced in the testicles of normal rats an acute orchitis such as resulted from inoculation of the blood or liver suspensions of splenectomized rats. While a few inclusions were found in the erythrocytes of some of the animals, their number was so small and their occurence so infrequent that they could not be definitely identified as B. muris. In morphological features the cultural forms of the microorganism isolated resemble B. muris. The organism found in the testicular tissues, however, is considerably coarser than B. muris and takes a deeper stain. Immunological study failed to settle the question of the relation between B. muris and the cultivated organism, which is provisionally called Bacterium murium. Bartonella muris, Bartonella bacilliformis, and Grahamella talpæ have characteristic individual morphological features.