The antispirochetal activity in vitro and in vivo of several antibiotics against ten isolates of Borrelia burgdorferi from human spinal fluids and skin biopsies was determined. Borrelia burgdorferi was most susceptible in vitro to erythromycin, ceftriaxone and cefotaxime (MIC90: 0.06, 0.06, 0.12 mcg/ml respectively). Less activity was observed with tetracycline, amoxycillin and lincomycin (MIC90: 0.50 mcg/ml), imipenem and augmentin (MIC90: 0.25 mcg/ml), oxacillin (MIC90: 1 mcg/ml), ciprofloxacin (MIC90: 2 mcg/ml) and ofloxacin (MIC90: 4 mcg/ml). Penicillin G, normally regarded as appropriate treatment for Lyme disease, had an MIC90 of only 4 mcg/ml. With the exception of erythromycin, activity in vitro corresponded to the activity in vivo. Erythromycin, however, was less active in vivo, and penicillin G showed poor activity both in vitro and in vivo.