Two rotavirus vaccine strains representing VP7 serotypes 1 and 2 derived by reassortment between a rhesus rotavirus master strain, MMU18006, and either of two human rotavirus strains were administered simultaneously to infants and young children to assess potential interactions between strains. Children were observed in a day care setting for 10 days after vaccine administration for clinical symptoms, evidence of vaccine transmission, and patterns of viral shedding. Serum and local antibody responses were measured. The ratio of input virus strongly influenced the amount of each strain recovered from the child. Regardless of dose of virus administered, the neutralizing antibody response to the VP7 glycoprotein, the serotype determinant, was diminished in a bivalent preparation compared with a monovalent vaccine. Additional strategies must be sought to induce immunity against the multiple serotypes of human rotavirus before broad immunity will be established.