Two clinical studies were conducted to identify the optimal dose of a modified tick-borne encephalitis (TBE) vaccine (FSME-IMMUN "new") in adults. A prospective, randomised, phase II, double-blind dose-finding study with the FSME-IMMUN "new" vaccine was performed in volunteers aged 16-65 years (n=405) to evaluate the immunogenicity and safety of two vaccinations with three vaccine doses (0.6, 1.2 and 2.4microg antigen). The safety and immunogenicity of the third vaccination were investigated in a follow-up study on the same study population. Antibody response to vaccination was assessed by enzyme-linked immunosorbent assay (ELISA) and, after the third vaccination, by neutralisation test (NT). Seroconversion rates (ELISA) in the different dose groups (0.6, 1.2 and 2.4 microg) were 85.1, 96.2 and 97.0%, respectively, after the second vaccination, which 73% of the volunteers received only 21 days after the first vaccination. Seroconversion rates after the third vaccination were 96, 99.2 and 100% (ELISA) as well as 77, 93 and 96.6% (NT) with the 0.6, 1.2 and 2.4 microg doses, respectively. No unexpected AEs or vaccine-related serious adverse events (SAE) were observed during either study. Local and systemic reactions were mainly mild and not dose-dependent, with an overall fever rate of <1% after the first vaccination. The 2.4 microg dose is the optimal dose for the FSME-IMMUN "new" preparation in adults, as it was found to: (1) be non-inferior to the 1.2 microg dose with respect to fever rate after the first vaccination; (2) induce the highest seroconversion rate; and (3) be well-tolerated with respect to local and systemic reactions. The results of both studies demonstrate that the FSME-IMMUN "new" vaccine is safe and highly immunogenic in adults.