This condition is fairly well recognized by veterinarians as occurring among domesticated animals, but in the records of this Garden it has occurred rarely, indeed only once in a mammal and but five times in birds. It is interesting that, in the wealth of material at the disposal of Plimmer and his associates, only one case, a polecat with lymphatic leucemia, is noted, and but very few avian instances. Herewith is submitted the protocol of our single mammalian case; perhaps we have missed others of a mild grade dying during the early stages because their resistance to infection was reduced. Unfortunately, perhaps because of the postmortem changes, but more likely because it appeared at first as if we had to do with a case of generalized tuberculosis, the bone marrow in this case was not examined. Nevertheless the infiltrative character of the lesions, the absence of distinct tumors and the numerous mononuclears in the blood as seen in sections seem to justify a diagnosis of leucemia, in all probability of the lymphatic type. There follows this case one with similar gross and microscopic picture which has no visible increase of leucocytes, but a very distinct myeloid picture in many places. Common Opossum (_Didelphys virginianus_). ♂ Adult. No evidence of illness. Found dead. Lymphatic leucemia (involving all viscera and lymph nodes). Diffuse nephritis. Both lungs have become entirely involved in a firm, gray-yellow mass not adherent to any serous surface. Practically no normal lung tissue is left. This seems like tuberculosis but no tubercle bacilli could be found in a good smear. Estimation of the normal cubic capacity of an opossum lung was made to be about five cubic inches. In this case not over one-half cubic inch remained respirable. Bronchial lymphatic glands were enlarged, firm, yellow gray, with no recognizable lymphatic tissue. Heart muscle was firm and flaccid, pale and striated. Liver very large, firm and tough, with smooth surface and sharp edges. Color pale brown. Section surface glistening, dry, smooth, opaque. Common bile duct patulous. Spleen, slightly increased, firm, tough consistency, capsule smooth. Section surface, smooth, firm, brown-red, pale pulp, prominent follicles, and trabeculæ faintly visible. Right kidney, slightly decreased, normal in shape. Capsule smooth, strips with difficulty, tears surface. Surface, granular, color brown, consistency firm. Thickness of cortex, narrow irregular, markings irregular and obscure. Small mass of fibrous material in cortex about 3 × 3 mm. like those in lungs. There are also numerous pale yellow gray areas in cortex and outer medulla, round and streaky, distorting the striated architecture. Right adrenal converted to a yellow gray mass like lungs. The mesenteric and retroperitoneal lymph nodes are firm, gray-yellow. This includes those under diaphragm and around cœliac axis.