Paneth cells occurring in intestinal and extraintestinal sites were studied using various light microscopic techniques, including standard hematoxylineosin (HE)-stained histologic sections; a number of histochemical reactions, including the phosphotungstic acid-hematoxylin (PTAH) method; and immunohistochemical labeling for the presence of lysozyme using the peroxidase-antiperoxidase (PAP) method. Cells, identified on HE as Paneth cells, uniformly stained with PTAH and PAP-lysozyme. Further, all of the cases demonstrating epithelial cell lysozyme showed eosinophilic cytoplasmic granules, typical of Paneth cells, on HE sections. Lysozyme was demonstrated in benign and malignant ovarian tumors, as well as in a variety of ectopic intestinal sites and benign and malignant intestinal tumors, supporting the concept that the elaboration of lysozyme is a nonspecific feature of intestinal-cell differentiation and is not a response to a specific stimulus.