An enzyme immunoassay (EIA) developed for the diagnosis of Lyme borreliosis was tested for its specificity and sensitivity in detecting IgG antibodies in patients at various stages of the disease. The EIA is based on a detergent extract of Borrelia burgdorferi which contains 12 proteins of defined molecular weights from Borrelia burgdorferi. The assay showed a specificity of 100% in control sera from 64 healthy individuals, using a cut-off optical density value of 0.13 (means +2- 3 SD). The sensitivity was 100% using sera from 22 Swedish patients with late stage Lyme borreliosis and 43% using sera from 30 patients with the initial stage of the disease. The reactivity of the sera against whole cell preparations, the outer surface proteins OspA and OspB, and the flagella of Borrelia burgdorferi was also tested and compared with the EIA. No cross-reactivity with treponemal antigens was observed when using the EIA.