The recent discovery of a repetitive element within the DNA of Mycobacterium tuberculosis, which is present in variable numbers at different locations in separate strains of the organism, has led to the development of genetic 'fingerprinting' to distinguish between different isolates. Clusters of cases of tuberculosis have been identified in Europe and the USA in which the organisms cultured had identical 'fingerprints' confirming that transmission was occurring. Unrelated isolates generally have distinct 'fingerprints'. In Africa, where transmission is more common than in Europe, there is less heterogeneity between isolates. We have typed 117 isolates of M. tuberculosis collected from continuing studies in Malawi and Kenya. Paired isolates from an individual patient produced matching 'fingerprints' in 22 of 25 cases. There were 18 isolates which had an identical matched pair from a separate patient; we have not yet found any epidemiological link between these patients. These data show that there is sufficient heterogeneity amongst African isolates of M. tuberculosis to make studies of transmission feasible and to address questions of pathogenesis and epidemiology.