A candidate live attenuated rotavirus vaccine representative of serotype 1 was administered orally to 26 children, 14 of whom were undergoing primary exposure to rotavirus. The vaccine was derived by reassortment between rhesus rotavirus strain, MMU 18006, and the human serotype 1 strain Wa. The resultant virus has the gene coding for the major surface glycoprotein VP-7 from the human strain and all other genes from the attenuated rhesus parent which is a serotype 3 strain. Prior natural exposure to rotavirus determined the infectivity and immunogenicity of the vaccine. Only two of 12 seropositive children had evidence of reinfection while all 14 seronegative children were infected. Mild febrile illness was seen in vaccinees, however there was no evidence of gastrointestinal disease. As determined by neutralization of the human strains, the resultant serum antibody was entirely strain specific. However, heterotypic neutralization was seen when the rhesus strains were used, suggesting that neutralizing antibody can be directed to shared components of the donor and reassortant strain presumably VP-4, the other major surface protein.