The transfer of rotavirus antibodies from 25 healthy mothers to their breast-fed infants was investigated during the period of lactation (mean, 3.9 months; range, 1-9 months). Furthermore, the destiny of these antibodies in the infants' gastrointestinal tract and serum was examined. Rotavirus-specific immunoglobulins were analyzed by the ELISA (enzyme-linked immunosorbent assay) technique. All the mothers had rotavirus IgA and IgG in serum. About 80% of the mothers had low concentrations of rotavirus ScIg (i.e., antirotavirus immunoglobulin containing secretory component) in serum at the beginning of the lactation period declining to about 45% at the end of the period. From a few days after delivery to about 2 weeks later, the concentrations of rotavirus IgA and ScIg in milk declined. Thereafter, they remained unchanged. There was a positive correlation among the concentrations of rotavirus IgA in serum and rotavirus IgA as well as ScIg in milk. Rotavirus IgG in the infants' serum correlated with that of the mothers. Few samples of the infants' duodenal fluid contained rotavirus IgA or ScIg. On the other hand, about 80% of the infants' fecal samples contained rotavirus ScIg and IgA. Rotavirus IgA and ScIg disappeared from the infants' feces after cessation of lactation. Hence, it may be concluded that infants receive rotavirus IgG through the placenta, and rotavirus ScIg and IgA in constant amounts via milk throughout the period of lactation. The small intestine is flushed with rotavirus ScIg and IgA at each breast-meal, and these antibodies survive proteolysis in the gut. A possible protectional effect of rotavirus ScIg or IgA requires frequent breast-meals, and the effect is limited to the period of lactation.