Human monocytic ehrlichiosis (HME) is a tick-borne infectious disease caused by the bacterium Ehrlichia chaffeensis and transmitted by the ixodid tick Amblyomma americanum. The first confirmed case of HME in Indiana occurred in 1994. Since then, there have been an additional 17 confirmed cases reported from 11 counties.A total of 498 A. americanum and 25 Dermacentor variabilis ticks were collected from counties in southern Indiana during May and June 1998, pooled, and examined for the presence of E. chaffeensis using nested PCR with primers HE 1 and HE3, which are specific for the 16S rRNA gene of E. chaffeensis. Ten pools of adult A. americanum specimens tested positive for E. chaffeensis DNA. This represented a minimum infection rate (MIR) of 3.82%. None of the A. americanum nymphs or adult D. variabilis ticks tested positive.In addition, 325 white-tailed deer blood samples from Indiana and 327 from Ohio were collected during November, 1998 and tested for the presence of E. chaffeensisreactive antibodies using an indirect immunofluoescence assay (IFA). Evidence of such antibodies was found in deer killed in six Indiana counties where infection rates ranged from 43% - 64% and four Ohio counties where infection rates ranged from 4% - 25%.The results from this study support the view that the distribution of E. chaffeensis closely follows that of A. americanum in the North Central United States. This is the first report of E. chaffeensis-reactive antibodies in white-tailed deer from Ohio. / Department of Biology