This book is a direct result of a symposium held in December 1988, in Washington, DC, honoring Professor Emeritus Miodrag Ristic for his contributions to rickettsial disease research, in general, and, to ehrlichiosis, in particular. He and his colleagues in the United States Army Medical Research Unit brought to the world's attention an epidemic of ehrlichiosis, that occurred in German shepherd dogs during the Vietnam War. The group was able to culture the microorganism Ehrlichia canis and to fulfill Koch's postulates. They eventually developed an indirect immunofluorescent antibody (IFA) test which has been adopted internationally. The same group joined a national effort to decipher another mysterious disease known as Potomac horse fever (PHF). They used the same technology developed by Nyindo in Dr. Ristic's laboratory to isolate, characterize, and again develop a similar IFA test for PHF. Today PHF has been diagnosed, at least serologically, practically across the entire United States, in some provinces of Canada, and reports are beginning to trickle in of its occurrence in European countries. Thus, the etiologic agent of PHF, now named after Professor Ristic, Ehrlichia risticii, historically places this scientist side by side with the other 2'R's', i. e., Ricketts and da Rochalima. Ehrlichiosis is not limited to domestic animals. Sennetsu rickettsiosis, long known by Japanese scientists as an imitator of'infectious mononucleosis,'was subsequently shown to be caused by an ehrlichial agent, through collaborative efforts among the United States Army, the University of Illinois, and Dr.