Blood sera of 221 healthy residents of 4 districts of Ryazan region were tested in the neutralization test (NT) with Tahyna and Inkoo viruses. The sum of positive responses in the region was 40.7% on an average, this indicating numerous contacts of the local population with these agents. Blood sera of 520 patients with acute seasonal (May-September) fevers and neuroinfections were tested for antibodies to Tahyna and Inkoo viruses in serologic tests (NT and MAC-ELISA-IgM-antibody capture method). Etiologic relationship of the diseases with Tahyna and Inkoo viruses was detected in 51 (9.8%) cases. The patients developed two types of specific immune response, primary or secondary. The best diagnostic result was attained in testing paired sera of patients in NT and MAC-ELISA. MAC-ELISA helped etiologically diagnose the disease after a single testing of a blood serum collected in the acute period of the disease. Inkoo infection was more incident in subjects aged 21 to 40 in May and July-August. The majority of cases of Tahyna infection were diagnosed in subjects aged under 20 in June and July. The incubation period was from 3 to 7 days, the morbidity was sporadic.