There have been repeated discussions as to whether the implementation of anti-HBc screening of blood donations in Germany would be useful. We present several cases of HBsAg-negative patients and blood donors in whose plasma HBV-DNA was mainly found only after enrichment of virus particles by ultracentrifugation. In the case of a seroconverting blood donor, 3 of 4 HBsAg tests could not detect HBsAg. 90% of our HBsAg-negative, but HBV-DNA containing samples (n = 10) were positive for anti-HBc. The 'nested' PCR without previous ultracentrifugation was positive in only under 30% of the samples. Ultracentrifugation is an expensive method and will not be practicable for the testing of all blood donations. We conclude that the HBsAg tests which are available should be subjected to improvement. In addition, the implementation of anti-HBc screening could decrease the risk of posttransfusion HBV infection more effectively than PCR testing from nonenriched serum.