During the first month of life 28 full-term newborns were breast-fed (18 males and 11 females). Thereafter 8 infants continued breast-feeding while the remainder were randomly fed on either an adapted milk formula (n=13) or a soy-formula (n=7). At five months, after an oral dose of RIT 4237 rotavirus vaccine of bovine origin was given, growth and IgM/IgG type antibodies against rotavirus were measured. Weight gain was similar in all infants. There were 2 IgM and 1 IgG responders out of 7 soy fed infants, compared with 4 out of 8 human milk fed (both IgM and IgG) and 7 out of 13 IgM and 6/12 IgG formula fed infants responding to vaccination. This observation confirms previous results obtained with polio, diphtheria tetanus and pertussis vaccines indicating that soy-protein formulas may interfere with immunization processes.