Fulminant hepatitis B developed in 8 recipients of blood units without detectable hepatitis B surface antigen on routine screening. All 124 hepatitis B virus (HBV) DNA clones propagated from their sera possessed defects in the precore region. A point mutation from guanine to adenine at nucleotide 83, converting codon 28 for tryptophan (TGG) to a stop codon (TAG), was the commonest, and it was found in all 113 clones from 7 cases. The remaining case displayed 1 clone with this point mutation and 10 clones with an insertion of 2 base pairs after nucleotide 26. Antibody to hepatitis B core antigen (anti-HBc) was detected in a high titer in 1 of 10 pilot plasma samples of blood units transfused to this case. HBV DNA clones propagated from it exhibited the same precore-region defects as those from the recipient. On the basis of these results HBV mutants, defective in the precore region, would appear to be responsible for posttransfusion fulminant hepatitis B, and the exclusion of blood units with high-titered anti-HBc would be efficacious in preventing it.