The acquisition of serum antirotavirus antibodies among children in day care centers was monitored through two rotavirus seasons. Twenty-six children were monitored daily for diarrhea and weekly for stool rotavirus excretion through a rotavirus season of infections with serotype G1 and a successive season of infections with both G1 and G3. Sera were collected before and after each rotavirus season and tested for antirotavirus IgA and IgG and for G type-specific blocking antibody. The prevalence of protective serum IgA and IgG titers increased from 36% and 45% before Season 1 to 77% and 96% after Season 2, respectively (P< 0.02 and 0.001). G type-specific antibodies also increased (G1,P< 0.001; G2,P= 0.005; G3,P= 0.003; G4,P= 0.006), including for noncirculating types. Homotypic and heterotypic antibodies increased as the number of rotavirus infections experienced by a child increased. The group of children with two proven infections developed protective isotype-specific and G type-specific antibodies. These results indicate that in first exposures to rotavirus G types, children develop predominantly homotypic antibody. However, as number of rotavirus infections increase, children develop heterotypic antibody to G types at levels that correlate with broad protection against rotavirus infection and illness, despite exposure to a restricted number of G types.