Solid-phase enzyme immunoassay was used to study changes in the frequency and levels of antibody to hepatitis A virus (anti-HAV) in 567 children in 20 isolated groups of 5 day-care centers of the town of Kemerovo. The observation period included the seasonal rise in hepatitis A incidence. An increase in the portion of seropositive children occurred in the groups where manifest or asymptomatic infection with hepatitis A existed. The number of seropositive subjects in the groups where the children were given immunoglobulin by epidemiological indications increased by 0-8%, and in those without immunoglobulin by 11-37.5%. The passive antibody received with immunoglobulin were detected in the originally seronegative children and declined to undetectable levels within 2 months. Subsequently, no clinical cases of hepatitis A or asymptomatic seroconversions were observed in these children. Some cases of reinfection showing an increase in anti-HAV levels but without clinical manifestations of hepatitis A were detected.