To study the heterogeneity of hepatitis B virus (HBV) DNAs in Hong Kong, where HBV infection is endemic, serum specimens from 90 HBsAg carrier children were systematically tested with nine oligonucleotide probes representing conserved sequences in the viral genome. In a pair of twins and their cousin (belonging to family H) and an unrelated child (family Y) serum HBV DNA annealed to all but one probe, a sequence located between the core and pre-S regions of the viral genome (positions 2723-2738; EcoRI site 1). Serum from the H twins' aunt and the father and paternal grandparents in the Y family were also HBV DNA-positive. The nucleotide sequences in positions 2701-2800 were analysed. The same point mutation, C to T in position 2735, was present in the HBV DNAs from 7 individuals in the two families. All 4 H family members had the same HBV DNA sequence. The HBV DNA sequences found in the Y daughter, father, and grandfather were identical and they were different from the H family mutant. These results provided evidence at the DNA level of intrafamilial transmission within these Chinese families.