Hepatitis B virus (HBV) with an in-frame stop codon within the pre-core region of the virus genome caused fulminant hepatitis in two individuals. Serum from a chronically infected patient who was the source of the virus was inoculated into three chimpanzees at dilutions of 10(-1), 10(-3), and 10(-7). All three chimpanzees developed acute hepatitis B with relatively high peak values of liver enzymes in their serum. The complete nucleotide sequence of virus DNA recovered from the chimpanzee serum by enzymatic amplification was identical with that from the human serum. By comparing the sequence of this strain (HT) with that of 32 published HBV genomes, changes in nucleotides and predicted amino acids that were rarely or never found in other HBV isolates were identified. Thirteen such nucleotides were found within the cis-acting regulatory elements, of which 6 were within the enhancer II-core promoter region. Twenty-four rare or unique changes in amino acids were found in open reading frames, of which 15 occurred in the region that spanned the 3' half of the X gene, through the pre-core/core gene, to the 5' end of the polymerase gene. Thus, an HBV pre-core stop mutant implicated in fulminant hepatitis is highly infectious, induces severe hepatitis in chimpanzees, and possesses significant genetic variation from reported HBV isolates.