We measured the levels of specific secretory IgA in human milk by using an enzyme-linked immunosorbent assay. We studied Giardia lamblia-infected and -uninfected mothers to determine the role of such IgA in providing protection against Giardia infection to their children. Titers of specific secretory IgA in the milk of infected mothers were significantly higher than were those of uninfected mothers (P less than .01). Only 16% of infants of mothers with high titers of antibodies (higher OD) in their milk were infected with Giardia. In contrast, there was an infection rate of 63.0% in infants of mothers with low titers of antibody in their milk (lower OD; P less than .01).