We report an outbreak of hepatitis A in nine South African hemophiliacs treated exclusively with infusions of factor VIII concentrate. The solvent/detergent (S/D) method (which does not inactivate nonenveloped viruses) was used for virus eradication. In three of six patients studied at a molecular level hepatitis A virus (HAV) complementary DNA (cDNA) amplified from serum by reverse transcription-polymerase chain reaction (RT-PCR) was identical to HAV cDNA in 1 of 14 lots (no. 53) of factor VIII tested. The plasma for lot 53 was obtained from donors in the United States: 1 of 12 samples of this source material contained the same HAV cDNA sequences. In another two patients receiving lot 53, the degree of identity between HAV cDNA in serum and that in factor VIII was only 88% and 89%, respectively, and in the sixth patient, who did not receive lot 53, it was 90%. The degree of HAV cDNA homology among five patients in an independent point-source outbreak of hepatitis A was 100%, and that in nine patients who acquired this infection sporadically in Johannesburg, Egypt, or Russia, was 93% to 100%. Whether the three hemophiliacs with low HAV cDNA homology with lot 53 acquired the infection from other unidentified virus strains contaminating factor VIII or from extraneous sources is unknown. This study provides further evidence that factor VIII concentrate prepared by the S/D method may be contaminated by the HAV, in this instance at source, and may cause hepatitis in recipients. Proof of a cause-and-effect relationship will, however, depend on animal inoculation studies. (Hepatology 1995; 22:1363-1367).