AbstractThe simultaneous demonstration of lysozyme using the unlabelled antibody enzyme method and mucosubstances by staining with the periodic acid‐Schiff (PAS) and Alcian Blue pH 2.5 techniques has led to the identification of a new subpopulation of mucus‐producing cells containing lysozyme in small intestinal specimens from normal rats and from patients with inflammatory bowel diseases. These cells, containing both mucosubstances and lysozyme, are located in the lateral walls of the crypts of Lieberkuhn, but can occasionally be found also in or near the tips of the villi. The specific staining for lysozyme was observed in the apical and/or basal cytoplasm of these mucus‐producing cells and was readily detected in sections counterstained for mucosub‐stances with Alcian Blue. The localization of these mucus‐producing cells was similar both in the normal rat and in the pathological human specimens. Absorption controls and controls where a non‐immune serum was substituted for the specific antilysozyme serum confirmed the specificity of the lysozyme l