IgM and IgG were detected in the sera from 25 Russian patients with Lyme disease by immunoblotting test with Immunetics kit (USA). Early stage of the disease was diagnosed in 12 patients and late stage in 13. Specific protein lines were detected in virtually all patients but their number and combination in the sera were different. Eleven (43%) sera were regarded as positive according to the American criteria of seropositive test, 3 of them by IgM and 10 by IgG. In patients with short disease serum reaction was either negative or confined to 1-2 lines per strip. Patients with late stage showed a more manifest extensive reaction, which was the most pronounced in patients with a long lasting disease and articular involvement; the reaction was similar to that characteristic of chronic Lyme arthritis. Parallel testing of IgG antibodies in 25 sera from American patients with Lyme disease (confirmed by direct isolation of the agent) showed notable similarity of reactions of both American and Russian sera. More essential differences were observed only for low molecular weight proteins (p28, p23, p21).