A histological examination of a biopsy from a firm submandibular mass in a seven-year-old domestic short-haired cat revealed a granulomatous lymphadenitis associated with the presence of small numbers of acid-fast bacilli. The cat was euthanased and subjected to a detailed post mortem examination which revealed extensive granulomatous inflammation in the right and left bronchial, para-aortic, mesenteric and colic lymph nodes, with small or early lesions in the lung and Peyer's patches of the ileum. Mycobacteria were isolated from the submandibular, mesenteric and cervical lymph nodes. The bacilli reacted with a DNA probe specific for the tuberculosis complex, including Mycobacterium tuberculosis and M bovis, but had cultural characteristics intermediate between these two species. The pathological findings are compared with previous reports of mycobacterial infections in cats, and the public health implications are discussed.