Five 8-week-old pigs were given an oral dose of approximately 10(8) viable units of Mycobacterium avium and confined in such a way as to prevent ingestion of fecal material. At necropsy 70 to 79 days later, there were granulomatous lesions, some containing acid-fast organisms, in the tonsils, aggregated lymphatic follicles (Peyer's patches) of the jejunum and ileum, mucosa of the caudal portion of the ileum, ileocecal valve, and cecum. The organism was isolated from tonsils, intestine, and swabs from the surface of some of these tissues. Mycobacterium avium that apparently orignated from lesions in the pigs was first detected in the feces 20 to 23 days after the animals were inoculated. The number of organisms excreted began to decrease at 55 days, and none was isolated from feces at 69 days. It was concluded that lesions in the intestine and possibly the tonsils were the sources of the organisms in the feces. The role of tonsil and intestine lesions of swine as sources of perpetual herd infections is discussed.