There is an enlarging spectrum of tick-transmitted spirochetal diseases. Lyme disease, a relatively new and clinically important syndrome is caused by Borrelia burgdorferi (a new species) which is transmitted primarily by the bite of Ixodes ticks. The number of human cases and the geographical area in which the disease is present is increasing in number and size, respectively. Recent reports indicate that the North American Borrelia, B. hermsii can cause chronic infection lasting 10 years with episodes of exacerbation. The specific aims of this proposal are: 1) Conduct genetic studies of the Borrelia 2) Determine optimal conditions for the isolation of the relapsing fever borrelia 3) Define host and bacterial factors involved in pathogenesis of borreliosis 4) Determine the in vitro and in vivo antibotic sensitivity of the relapsing fever borreliae and 5) Development of test for serodiagnosis of borreleosis.