In June 1938 the writer obtained three rats from Dr Gye and Dr Knox which had been previously infected with the “Pyogenic Virus” received by them from Dr Woglom (Woglom & Warren, 1938a). Pus from these rats was spread on plates of the writer's “special medium” for the isolation of pleuropneumonia-like organisms. No bacterial organism had been detected in the pus which, however, was full of granules and showed some filaments in the dark-ground preparation; after 6–10 days of incubation the plates were crowded with tiny colonies. They proved to be colonies of a pleuropneumonia-like organism, and the finding was recorded in an appendix to a recent paper by the writer (Klieneberger, 1938). At the edge of the growth from the seeded pus, where unhampered development was possible, the colonies showed a granular structure with a well-marked dense centre (see Fig. 1). No other organism was isolated from the lesions. More rats were now infected from the pus and all developed abscesses in the way described by Woglom & Warren (1938a, b). From these lesions the same culture was obtained (to be referred to as “Woglom strain I”). Since June 1938 up to date the pyogenic agent obtained from Dr Gye (to be referred to as “original agent”) has been transferred from rat to rat by subcutaneous injection of the crude pus. So far, forty-five rats have been infected with this agent; they all developed abscesses, and the pleuropneumonia-like organism was invariably isolated from the pus. In thirteen of these rats spleen culture was attempted, and the pleuropneumonia-like organism was obtained from nine out of these thirteen spleens. If the spleen culture was positive, subcutaneous injection of a suspension of spleen produced an abscess, while a culturally negative spleen had no effect if injected.