Human-rhesus reassortant rotavirus vaccines derived from rhesus rotavirus and human rotavirus serotypes 1, 2, or 4 were administered to adults, children, and infants after they had been given sodium bicarbonate buffer. Over 70% of infants and children developed antibody responses to or shed the candidate vaccine viruses. Individuals with prevaccination serum neutralization antibody titers less than or equal to 1:160 had significantly (P less than .0001) higher response rates than did individuals with prevaccination titers greater than or equal to 1:320. Similarly, shedding of vaccine viruses occurred significantly (P = .03) more often in children than adults and was inversely correlated with prevaccination antibody titers. No illnesses were observed in adults, and the vaccines were well tolerated in children. An increased rate of low-grade, transient fever (38.0-38.8 C) was noted only in children given serotype 2 vaccine, compared with controls (P = .006). No significant differences in the average number of unformed stools passed during the seven days after vaccination were noted in vaccinees versus controls. These vaccine strains are currently undergoing further evaluation in expanded clinical trials.