Plasmid profile analysis was performed on Mycobacterium avium/M. intracellulare isolates from patients with AIDS, from children with cervical lymphadenitis, and from environmental sources. The frequency of plasmid-containing strains was found to be, respectively, 5/16 (31%), 4/15 (27%) and 1/15 (7%). The data indicate that plasmids occur more frequently among clinical isolates than among environmental isolates, and support a possible pathogenic role of the plasmids.