A sonicate antigen and two concentrations of a purified flagellum antigen of Borrelia burgdorferi were compared for serological diagnosis of Lyme borreliosis by an enzyme immunoassay (EIA). Generally, the higher concentration of flagellum antigen was found to be superior to the lower concentration, which was diluted eight times compared to the higher concentration. The diagnostic sensitivity for IgG antibody detection increased from 13% in the sonicate EIA to 31% in the best flagellum EIA assay (p = 0.01) in sera from patients with erythema migrans (n = 70), and from 34% to 55% (p = 0.01) in sera from patients with neuroborreliosis (n = 77). However, the sensitivity for IgG in sera from patients with acrodermatitis chronica atrophicans (n = 20) was high in both assays: 90% in the sonicate EIA compared to 95% in the flagellum EIA. Regarding IgM, there was no significant difference between the sensitivity of the assays in sera from any of the patient groups. The sensitivity values for IgM and IgG in cerebrospinal fluid (CSF) from patients with neuroborreliosis were also without significant differences. Sera and CSF from patients with meningitis/encephalitis of non-Borrelia etiology (n = 35), multiple sclerosis (n = 9) or syphilis (n = 24), served as controls. The flagellum EIA showed a significantly improved specificity for IgG in CSF from controls with syphilis (p less than 0.01). It is concluded that purified Borrelia burgdorferi flagellum antigen is superior to a sonicate antigen, especially for serodiagnosis of the early stages of Lyme borreliosis.