The presence of “bodies” within the large mononuclear leucocytes of guinea-pigs was first noticed by Kurloff (1898). He described them as inclusions; for in a drop of guinea-pig’s blood he noted that many of the large lymphocytes contained, within their cytoplasm, clear, spherical vacuoles which were distinct from the nucleus, and which had not been described before; and he suggested the possibility of these bodies being accessory nuclei. Since their discovery by Kurloff they have been subjected to much research; and papers describing various observations concerning them have been published by Burnett (1904), Staubli (1905), Goldhorn (1905), Ledingham (1906), Howard (1907), Pappenheim (1908), Patellle (1908), Hunter (1909), Schilling (1911). Kurloff noticed that when the blood containing these bodies was fixed and stained, they contained a nucleus-like structure staining with nuclear dyes, but he belived them to be vacuoles formed by a secretion product of the cells which held them. Ehrlich (1906) also thought that Kurloff’s bodies represented some “Secretstoff.” Dr. Ledingham, to whom I am indebted for much information, seems to have been the first to suggest the possibility of their parasitic nature, and he mooted an analogy to the Cytorrytes variolae or vaccinae . Goldhorn (1905) boldly called them leucocyctozoa. The most recent work published on the subject is that of Schilling (1911). He has examined these bodies by “vital” staining with Azur, and he has described some of the earlier stages of their development while in the mononuclear leucocytes (lymphocytes). He believes that the rod stage precedes the granule stages, and this has caused him to adhere to the opinion that Kurloff’s bodies must be classed with the Chlamydozoa, symbiotic structures, or vaccine inclusions.