The immunological finding "anti-HBc alone" (without HBsAg, without anti-HBs) leaves open the question of the state of HBV infection it reflects: a transitory stage of an uncomplicated, eventually prolonged but resolving infection, a chronic or a late state of immunity. A finding of this kind is often observed in immunocompromised individuals (e.g. patients on hemodialysis, drug addicts) but also occurs in up to 1% of the Swiss blood donor population. Of 8800 sera tested for HBV marker in a diagnostic laboratory, 153 individuals showed "anti-HBc alone". They were investigated for circulating hepatitis B desoxyribonucleic acid (HBV-DNA) by polymerase chain reaction (PCR). 60 individuals (39%) showed a positive result. Also taking into consideration anamnestic measurements of conventional HBV markers in 95 individuals and consecutive testing for HBV-DNA in 50 individuals, the following conclusions emerged: 1. A positive finding of HBV-DNS by PCR does not necessarily prove an ongoing HBV infection, hence a negative result does not rule it out. Therefore, the indication to test for this parameter is limited for routine use. 2. The finding of "anti-HBc alone" implies that a HBV-infection is still going on until proven otherwise. This not only might be of consequence for the individual involved, but also raises the question of screening of blood donors and of pregnant women for anti-HBc.