Through the investigations of Noguchi on the one hand, and Mayer and Kikuth on the other, the unity of the aetiology of Oroya fever and verruga peruviana has been demonstrated experimentally. The Bartonella bacilliformis, the causal organism of Oroya fever, belongs to a group of micro-organisms, the parasitic nature of which has been definitely proved during the last few years. Tests with a view to transmitting the disease to monkeys, and the aetiology, clinical data, pathology, therapy and immunobiology are further explained with reference to personal experiments.Bartonella muris, which was first observed by Mayer in 1921, was confirmed by Mayer, Borchardt and Kikuth to be the causal organism of infectious rat anaemia following splenectomy. After splenectomy of the rat, the latent parasite becomes virulent and often causes a fatal anaemia. The clinical course of this infection is connected with an endothelial reaction which can be demonstrated histologically. The infection is transmitted by rat lice. Chemotherapeutic experiments led to the discovery of an effective arsenic-antimony compound with an index of 1: 3,500, which figure has never hitherto been reached in chemotherapy.The causal organism of dog anaemia following splenectomty is the Bartonella canis.Bartonella and bartonella-like structures as causal organisms and harmless blood parasites in various animals.On account of their peculiar behaviour the Grahamella, which were first described by Graham-Smith, should be kept strictly apart from the Bartonella and looked upon as a species by itself.