A total of 522 children between 18 months and 14 years and 191 adults between 18 and 60 years were vaccinated with TBE-vaccine according to an abbreviated schedule (0, 7, 21). The aim of the study was to investigate whether reducing the amount of antigen in the vaccination for children would preserve an adequate immune response and decrease the rate of side-effects. Efficacy was determined on the extent to which children, vaccinated with the low doses (0.4 microgram or 0.75 microgram), reacted by developing antibodies in the same way as adults treated with the approved dose of 1.5 micrograms (equivalence of titres). The titres obtained in the children with the two lower doses were equivalent to those in the adults obtained with the standard dose. Titres decreased in the children with increasing age. Children older than 12 years in the approved dosage group had the same median titres as adults. The frequency of side-effects in the two lower dose groups, especially raised temperature, was markedly reduced. Whereas 30.1% of the children vaccinated with the approved dose had raised temperature higher than 38 degrees C only 18.8% and 18.4%, respectively, of the children vaccinated with the lower doses developed such temperatures. This improved tolerance in terms of raised temperature was also reflected in the other general reactions such as tiredness, joint pain or headache.