Scanning electron microscopy (SEM) studies were performed on freshly prepared and freeze-dried Tice TM-substrain Mycobacterium bovis BCG vaccine as well as Tice BCG grown on Middlebrook 7H10 agar. Intact colonies of the Tice and Glaxo BCG substrains growing on agar were also examined. The presence of developmental stages of the mycobacterial life cycle previously reported in the literature was confirmed in actively growing BCG and in commercial vaccine preparations. The pleomorphic forms consisted of various size coccal and bacillary cells. Propagation appeared to occur by fission of both forms to produce aggregate bodies and by a coccal-bacillary cycle. Filterable (30-200 nm) granular cocci and coccal microcolonies were also observed in commercially prepared BCG vaccines. The implications of pleomorphism on the biologic activities of various BCG vaccines are discussed.