We prepared fractions of Borrelia burgdorferi, the etiologic agent of Lyme disease, from cultured spirochetes and used them as antigen in an enzyme-linked immunosorbent assay (ELISA) for IgG antibody. Polystyrene plates coated with an extract containing major proteins with apparent molecular masses of 34, 39, 59, and 68 kilodaltons had comparable sensitivity but greater specificity than plates coated with whole cells. Of the 33 serum specimens from individuals with Lyme disease that reacted with whole cells of B. burgdorferi in the class-specific ELISA, 30 (91%) remained positive when this extract was used. Cross-reactivity was minimal with antibody to treponemes. Use of subunit antigens may improve serological diagnosis of Lyme disease.