Twenty five children with diarrhea were studied. Ten children without diarrhea or that had the last episodic one year before biopsy, represented the control group. Jejunal juice and biopsy specimens obtained from each patient were cultured, and bacteria obtained from these were tested against a further piece of mucosa and sample of pure juice from the same patient, to determine the presence within them of antibody against the small bowel bacteria. Duodenal mucosae antibody against non-pathogenic bacteria was demonstrated in 95% of children with diarrhea and only 25% of control group. The immunoglobulin classes involved, IgA and IgM, suggest a strong local immune system stimulation, although a systemic immune response can not be excluded since IgG was frequently found in the mucosa.