The antimicrobial susceptibility of five Lyme disease spirochete strains (two human and three tick isolates) was determined. A macrodilution broth technique was used to determine on three separate test occasions the minimal inhibitory concentrations (MICs) of seven antibiotics. The Lyme disease spirochete was most susceptible to erythromycin with a MIC of less than or equal to 0.06 micrograms/ml. The spirochete was also found to be susceptible to minocycline, ampicillin, doxycycline, and tetracycline-HCL with respective mean MICs of less than or equal to 0.13, less than or equal to 0.25, less than or equal to 0.63, and less than or equal to 0.79 micrograms/ml. The spirochete was moderately susceptible to penicillin G with a mean MIC of 0.93 micrograms/ml. All strains were resistant to rifampin at the highest concentration tested (16.0 micrograms/ml).