An antiserum prepared against lysozyme isolated from mucosal scrapings of mouse small intestine was used to stain sections of mouse small intestine with the indirect fluorescent antibody technique. Mucosal fluorescence was confined to the base of the crypts of Lieberkuhn, where Paneth cells are located. After the intravenous administration of 4 mg of pilocarpine fluorescence was no longer found in the Paneth cell but in the crypt lumen. Perfusion studies confirmed these findings. The basal lysozyme output of 0-1 to 0-4 mug/ml was raised to peak rates of 1-8 to 6-5 mug/ml after the intravenous administration of 1 mg of pilocarpine. Our results demonstrate that the lysozyme of the succus entericus is, at least in part, derived from the Paneth cell, and is probably present in the Paneth cell granules. Its secretion is stimulated by pilocarpine. Our model could be very useful for studying the function of the Paneth cell, which probably forms part of an intestinal defence system.