Human milk obtained from 21 American nursing mothers was studied for the presence of secretory immunoglobulin A antibody to rotavirus, the most common etiological agent of infantile gastroenteritis. Antibody was quantitated by adaptation of a recently described solid-phase radioimmunoassay technique that employs simian rotavirus as a convenient substitute antigen for human rotavirus. Of the mothers tested, 80% (12 of 15) possessed milk antibody within a week of parturition, whereas 56% of those tested (5 of 9) secreted milk antibody as late as 6 or 9 months postpartum. Specificity of the radioimmunoassay was demonstrated by absorption of antibody with purified rotavirus. Our detection by radioimmunoassay of antibody to rotavirus in milk samples collected past the colostrum stage is in contrast to other studies that have failed to detect antibody in human milk by immunofluorescence or neutralization tests. The present study also suggested that the appearance of secretory immunoglobulin A antibody in the milk of mothers previously lacking milk antibody may be correlated with subclinical infection of the mother with rotavirus.