Rotavirus vaccine strain M37 (serotype 1), recovered from the stool of an asymptomatic newborn infant and serially passaged in cell culture, was given orally to adults, children, and infants. Serologic responses were detected by neutralization assay or EIA in 59% of 17 adults (10(5)-pfu dose), 55%-60% of 21 infants and children (10(4)-pfu dose), and 70% of 10 infants (10(5)-pfu dose), vaccine virus was shed by 24%, 20%-36%, and 70%, respectively. In adults, neutralizing antibody rises to strain M37 and the related serotype 1 strain Wa occurred with equal frequency (41% vs. 47%). In pediatric subjects, the former were more frequent (36%-40%) than the latter (10%-18%). This was also true of 8 infants who received two doses of vaccine. Mild gastrointestinal illnesses occurred with equal frequency in pediatric subjects who received vaccine or placebo. Thus, strain M37 was well tolerated and immunogenic in young infants, but elicited primarily vaccine-strain-specific rather than serotype-specific neutralizing antibody responses.