Seronegative volunteers (15) were vaccinated at three 1-month intervals with a combined hepatitis A and B vaccine. The vaccine contained a killed hepatitis A vaccine made from hepatitis A virus (HAV) propagated in diploid human fibroblast cell cultures and hepatitis B surface antigen (HBsAg) produced in yeast. After only one injection, all volunteers developed neutralizing antibodies for HAV with antibody titers comparable to those found after gamma globulin administration. Compared with the antibody titers in sera of volunteers vaccinated with HAV vaccine alone, the antibody titers against HAV were significantly higher in the sera of the volunteers simultaneously vaccinated against hepatitis B virus (HBV) after three vaccinations. Of 15 volunteers, 14 seroconverted against HBV after three injections. In 11 volunteers tested 48 weeks after vaccination, antibody titers against HAV and HBV remained high.