We have employed a single phase antibody radioimmunoassay to determine rectal tissue and secretion immunoglobulin (Ig) levels in children with documented infectious colitis. All children had positive stool culture and/or rectal swab for pathogen, abnormal proctosigmoidoscopy and pathologic diagnosis of acute colitis. Tissue was placed in rectal organ culture for 24 hours and explants were homogenized (H) and assayed for IgA, IgG and IgM. Secretions (S) were collected over center well of organ culture plate and assayed for Ig's. Results were expressed as μg Ig per mg tissue protein. Results were compared with a control population of children with Functional Abdominal Pain who had normal proctosigmoidoscopy and pathogen-negative stool cultures. There appears to be a marked symmetrical increase in rectal tissue Ig's whether comparing total Ig (p<0.005) or individual homogenates (p<0.001) or secretions (p<0.01). The presence of increased amounts of Iga, IgM & IgG in infectious colitis should influence further investigations into other inflammations of the colon (i.e., ulcerative colitis, cow's milk protein-induced colitis. etc.).