In order to study whether and how M. bovis BCG can persist in vivo, series of events during interaction of live BCG bacilli with resting or preactivated with Lentinan peritoneal macrophages in guinea pigs were evaluated. After intraperitoneal administration of BCG, samples of peritoneal lavage fluid from guinea pigs were obtained at day 1, 14 and 45. Interactions between BCG bacilli and peritoneal macrophages were observed by transmission electron microscopy. Examination of some BCG bacilli inside macrophages revealed fate and morphological peculiarities typical of cell wall deficient bacterial L-forms as well as different modes of L-form multiplication. In correspondence to the observed L-form conversion phenomenon, BCG L-form cultures were isolated from animals on 45 day after challenge. The demonstration that BCG bacilli can convert to cell wall deficient forms(Lforms) inside resting or pre- activated macrophages in vivo suggests that this phenomenon could significantly enhance BCG survival and persistence ability.