AbstractHepatitis B virus infection is preventable by vaccine and the correlate of immunity is the induction of antibody. The 2.5 μg antigen per dose of H‐B‐VAX II given in appropriate regimens is adequate to immunize normal newborn infants and children up to 11 years of age. A 5 μg dose may be needed to protect newborn infants born to mothers who are carriers of hepatitis B infection and should be used when the carrier status is either positive or is unknown. The immune response in newborn infants is usually of lower titre than in infants who are 2–3 months of age or older. The level of protection afforded in otherwise healthy individuals by plasma‐derived vaccine is high and lasts for longer than 7 years, even though the level of circulating antibody may have declined to a less than detectable amount. A similar pattern of antibody decline follows administration of recombinant vaccine. The mechanism for sustained protection is in long‐term immunological memory with rapid recall of antibody production on exposure to the virus in nature. Administration of hepatitis B vaccine will be simplified by combination with DPT and poliomyelitis vaccines in single‐dose formulations that are under current development.An experimentally killed hepatitis A virus vaccine prepared by formaldehyde inactivation of attenuated live virus purified from infected MRC‐5 cell cultures induces rapid and high level antibody response when the vaccine is given in as little as 100 ng of antigen per dose and in two‐ or three‐dose regimens. The vaccine is highly protective when assayed in challenge studies in animals and the levels of antibodies achieved are far in excess of those provided by immune globulin when used for passive protection in humans. Field investigations to demonstrate protective efficacy against natural challenge in human beings have been in progress and protective efficacy h