Human ehrlichiosis is a newly recognized disease characterized by fever, headache, malaise, thrombocytopenia, leukopenia, and elevated liver enzymes (Anon., M.M.W.R.37, 270, 275, 1988; Fishbein, et al., JAMA257, 3100, 1987; Fishbein, et al., J. Infect. Dis.160, 803,1989;Eng, et al., JAMA264, 2251, 1990). Often the patients also have a history of tick exposure. The only Ehrlichia species known to infect humans is Ehrlichia sennetsu, the agent responsible for sennetsu rickettsiosis, a disease that has been reported only in Japan and Malaysia (Ristic, in Microbiology1986, L. Leive, Ed., American Society for Microbiology, Washington, D.C., 1986, pp. 182-187). Since recognition of a human form of ehrlichiosis in the United States in 1986, laboratory-based surveillance has led to the identification of about 215 persons with variable antibody titer to E. canis in 20 states, predominantly in southeastern and south central areas of the United States (Fishbein, et al., J. Infect. Dis., 160, 803, 1989; Eng, et al., JAMA264, 2251, 1990). It may be noted, however, that despite such serologic evidence, the causative agent of human ehrlichiosis remained unidentified and the etiology of the disease also remained undetermined.