We studied the prevalence of antibody to rotavirus in 386 serum samples obtained from different age-groups of infants in Bochum, West Germany. Cord sera mirrored the IgG titer and serotype specificity of the corresponding maternal sera. IgG antibody prevalence decreased with age, reached a minimum between four and 12 months, and increased thereafter. IgM antibodies appeared first in the eight- to 12-month age-group, and IgM prevalence remained high thereafter. The serum level of IgA was low in all age-groups. Significant prevalence increases with increasing age were observed for neutralizing antibodies to rotavirus serotypes 1 and 3. Antibodies to serotype 4 had not reached adult level at 2.5 y of age. Prevalence of antibodies to serotype 2 was low in all age-groups. The percentage of monospecific sera neutralizing only one serotype was high in young infants eight to 12 months of age and decreased significantly with increasing age.