Isolates of Mycobacterium tuberculosis from 88 patients with pulmonary tuberculosis in northern Tanzania were subjected to IS6110 restriction fragment length polymorphism (RFLP) analysis. Of 88 isolates, 73 fell into 11 groups of which 9 contained 2-5 isolates. Of 2 large homology groups one, group H (20 isolates), was isolated only from patients resident in the Kilimanjaro region, whereas 79% of isolates from other groups came from this region. A significant association (P = 0.023) was found between another group, M (24 isolates) and isolation from patients of the Masai tribe. The data from this pilot study support the idea that IS6110 RFLP analysis provides information which may be of value in the control of tuberculosis in Africa.