Six hundred and fifteen Swedish soldiers (X = 24.9 years) serving with the UN forces in Cyprus were tested for antibody against hepatitis A (anti-HAV) before and after six months' service. Before the service, a high susceptibility to hepatitis A was found, as only 2.3% (14/615) had anti-HAV. In comparison, the prevalence of anti-HAV among 124 native Cypriots (X = 23.9 years) was 97% (120/124), which showed Cyprus to be a region where hepatitis A is endemic. The soliders were given immunoglobulin (IG) prophylaxis (2 ml of 16.5% IG) against hepatitis A very third month. The incidence of subclinical seroconversion against hepatitis A among the Swedish soliders was 0.2% (1/615). It was concluded that the IG-prophylaxis scheme used seems to be highly effective and probably completely prevents even subclinical seroconversion against hepatitis A in non-immune persons in regions where hepatitis A is endemic.