Atypical mycobacteria are responsible for systemic infections which occur late in the course of AIDS. The Mycobacterium avium intracellulare complex is the most frequently blamed species. Clinical and laboratory data are not specific; they consist of protracted fever, gastrointestinal and respiratory disorders entangled with the numerous manifestations of the multiple pathologies observed in these patients. The diagnosis is ascertained by the isolation of mycobacteria from blood cultures (using the isolator lysis-centrifugation and the Bactec 6B system) and from tissue cultures (hepatic parenchyma, bone marrow, etc.). Pathological examinations of specimens obtained from liver needle biopsy, duodenal biopsy and myelogram are very useful in providing a more rapid diagnosis.