From ticks of the type Ixodes ricinus, 19 strains of a spirochete were isolated at three places of infection of erythema chronicum migrans disease. The spirochete was immunologically related to Borrelia duttoni, Treponema pallidum and Ixodes dammini spirochete, the causative organism of North American erythema chronicum migrans disease (Lyme disease). The isolated spirochete differed from the North American one in its reaction with monoclonal antibodies and possibly in its electronmicroscopic structure. A corresponding spirochete was isolated from the blood of a woman with erythema chronicum migrans. Of 39 patients with erythema chronicum migrans mostly treated with antibiotics 50% had increased IgG antibody titre (1:64 to 1:1024) against the isolated spirochete, while among 51 untreated patients with tick-transmitted meningopolyneuritis 90% had increased IgG antibody titres. Fourfold antibody titres increases or falls were found on 50 occasions. IgG antibody titres up to 1:64 were demonstrated also in CSF, in 22 instances with significant changes. Increased serum IgM antibody titres of 1:32 to 1:256 were observed in 20% and 68%, respectively, of patients. These findings suggest that the isolated spirochete is the causative agent of erythema chronicum migrans disease in Europe. Its antigen structure and arrangement is similar to that of the causative agent of Lyme disease.