The immunoserological finding "anti-HBc alone" is often observed in defined groups of individuals, such as patients with inflammatory hepatopathies, patients on hemodialyses or with organ transplants, i.v. drug users and homosexuals, but it also occurs in up to 1% of Swiss blood-donors. In order to gain further information about whether "anti-HBc alone" reflects late immunity or points to an ongoing or a recently passed hepatitis B virus infection, 153 serum samples were tested for immune-complex-dissociated HBs-antigen, using acid treatment for complex dissociation. Of the samples tested 31% contained complexed HBsAg, the highest rates being found in individuals with hepatopathies (up to 80%), in i.v. drug users (up to 63%) and in hemodialysis patients (40%). The 153 sera were also tested for HBV-DNA by nested PCR. Sixty (39%) probes yielded positive results, comprising 29 (48%) of 60 sera with immune-complexed HBsAg but only 18 (19%) of 93 probes without complexed HBsAg. The results point to the possibility that at least some of the individuals with "anti-HBc alone" still have an ongoing HBV-infection.