The safety of a plasma-derived hepatitis-B vaccine inactivated by two heating steps (90 sec at 103 degrees C followed by 10 hr pasteurization at 65 degrees C) was validated in chimpanzees; 10(3) chimpanzee-infectious doses (CID50) of hepatitis-B virus (HBV), subjected to the purification steps during production of the vaccine, were noninfectious in two chimpanzees. Furthermore, 10(6) CID50 of HBV heated at 103 degrees C for 90 sec and another 10(6) CID50 of HBV heated at 65 degrees C for 10 hr, were also not infectious in two other chimpanzees. All animals developed hepatitis-B infection after subsequent challenge with the untreated starting material, before the respective purification and inactivation procedures. Thus, the total reduction of infectivity of HBV during production of this vaccine was established to be at least 10(15) CID50.