Rotavirus vaccine strains representing serotypes 1 and 2 have been derived by reassortment between genes of a rhesus rotavirus master strain, MMU18006, and the gene from human viruses coding for VP7, a surface protein with neutralizing determinants. As simultaneous administration of these two human rotavirus reassortants had resulted in disappointing strain-specific immunity, these two vaccines were administered sequentially to infants and young children to assess whether immunity to both serotypes could be achieved with two monovalent doses. Following initial immunization with RRVxDS-1, a serotype 2 vaccine, 12 of 13 shed virus and showed serologic responses to multiple rotavirus proteins. However, with subsequent administration of RRVxD, serotype 1, only 2 of 12 shed virus, and an enhancement or broadening of the immune response was not uniformly seen. Although rhesus rotavirus vaccines are immunogenic with primary administration of monovalent vaccine in seronegative children, and supplemental seroresponses are seen with a second dose, other strategies or new vaccine candidates must be sought to induce immunity against the multiple serotypes of human rotavirus.