A total of 2784 cases with persistent symptoms of lower respiratory tract infections were seen in four of the chest clinics in Lagos between January and June of 1983. Of these, 668 were randomly selected and screened for pulmonary tuberculosis. Repeated sputum samples from the 668 patients were cultured on Lowenstein-Jenseen slopes and 102 pure mycobacterial isolates were obtained. The isolates were differentiated into tubercle bacilli, bovine or atypical mycobacteria on the basis of results obtained from nine identification tests. Of the 102 mycobacterial isolates obtained, 87 (85%) were identified as Mycobacterium tuberculosis, four (4%) as M. bovis and 11 (11%) as atypical mycobacteria. Among this atypical group, six were classified as M. avium, four as M. kansasii and one as M. fortuitum. The study showed the involvement of bovine and atypical mycobacteria in pulmonary infections in Lagos. Tuberculosis-like diseases produced by some of the atypical group are resistant to most of the conventionally used anti-tuberculosis drugs. The need for adequate bacteriological analysis in current-day diagnosis and treatment of pulmonary tuberculosis is therefore highlighted since the 11 atypical cases seen in this study would otherwise have been diagnosed and treated as cases of classical tuberculosis.