In highly industrialized countries certain groups of the population are at increased risk of infection and might be considered for targeted use of hepatitis A vaccines. Low levels of hepatitis A virus (HAV) were detected by cell culture in the feces of both animals that received variant F for 2 weeks prior to development of anti-HAV. The antigenicity of the vaccine was studied by injecting graded doses into groups of 5-week-old mice and young 5 labiatus marmosets. To prepare this vaccine, a rufiventer marmoset was infected i.v. with the Costa Rican strain (CR326) of HAV which had previously been passed 25 times in marmosets. Alternative viral vaccines can now be produced by the application of the relatively new disciplines of recombinant DNA technology. For the vaccine to have a major impact in developing countries it will need to be widely administered early in life, either as part of the routine childhood immunization program or as a prerequisite to school entry.