32 volunteers who lacked antibody to hepatitis A virus (HAV) received, at intervals of one month, three injections of a killed vaccine made from HAV propagated in diploid human fibroblast cell cultures. In 16 the vaccine was coupled with aluminium hydroxide as adjuvant. The serconversion rates measured by radioimmunoassay, with and without adjuvant respectively, were 13% and 25% at four weeks, 81% and 89% at eight weeks, and 94% and 100% at twelve weeks. Four weeks after the third injection all volunteers had high neutralising antibody titres. Twelve weeks later the titres of anti-HAV and anti-HAV neutralising antibodies had not declined.