The reactogenicity and antigenicity of the rhesus rotavirus vaccine, strain MMU18006, developed at the Laboratory of Infectious Diseases (National Institute of Allergy and Infectious Diseases, National Institutes of Health) were examined in a double blind, placebo-controlled study of 40 newborn infants in Caracas, Venezuela. The children were observed for the first few days after birth in the hospital nursery and by home visits for 10 days after vaccination to detect any adverse reactions. No reactions could be attributed to the vaccine. Serologic responses to the vaccine were evaluated in paired sera obtained at birth (cord blood) and 4 weeks after vaccination. Serologic responses to the vaccine were not observed by complement fixation, neutralization or a rhesus rotavirus VP7 epitope-specific competition assay. However, such responses were found in 9 of 14 tested infants by an immunoglobulin A-specific enzyme-linked immunosorbent assay. Seventeen of the 20 vaccinees also shed rhesus rotavirus vaccine in stool during the postvaccination period.