AbstractA sensitive enzyme immunoassay is described for demonstrating specific antibodies of the IgM class to tick‐borne encephalitis virus (anti‐TBEV IgM). Anti‐m̈‐coated, flat‐bottomed microtiter plates are incubated with diluted patients'serum (2 hr at 37°C), then with purified TBEV, and later with pero‐xidase‐coupled anti‐TBEV immunoglobulin for a further 2 hr. After washing the plates, orthophenylenediamine is added and the optical density is measured at 510 nm. At an early stage after onset of illness anti‐TBEV IgM could be demonstrated up to serum dilutions of 10−4. The specificity of the test system was shown by total inhibition of the positive reaction by preincubation of patients'sera with anti‐m̈. Tick‐borne encephalitis (TBE) could be diagnosed serologically in 54 patients by the described method, and anti‐TBEV IgM could be demonstrated in sera drawn up to 9 months after onset of illness. A peak in the number of infections occurs in the age group of 31–40 years, and most infections occur from June to Oct