Borrelia burgdorferi, the causative agent of Lyme disease, was detected in patients' serum by DNA amplification using the polymerase chain reaction (PCR). B burgdorferi was pelleted from serum samples by centrifugation (10,000 x g for 10 minutes) and lysed by treatment with ammonium hydroxide (100 degrees C for 15 minutes). Two pairs of "nested" PCR primers complementary to the gene encoding a major outer surface protein (OSP A) of B burgdorferi were used in DNA amplification under standard PCR conditions (Perkin-Elmer Cetus). Two out of five patients with erythema migrans, the characteristic primary skin lesion associated with early Lyme disease, were positive by the PCR. This method could form the basis of a useful routine laboratory test in those cases of early Lyme disease where conventional serological testing commonly yields equivocal or false negative results.