Mycobacterium marinum has been recommended as a possible model of M. leprae for use in laboratory studies of antileprosy immunity. M. marinum introduced into the footpads of normal mice underwent a steady decline in viability, with less than 1% survival after a 30-day period. Small numbers of viable bacilli were recovered from the footpads of these mice up to 12 months later. Similarly, mice infected with M. simiae exhibited bacterial populations that persisted for up to 18 months with little change in viability. Injection of M. simiae into the footpads was followed by an extensive redistribution of the organisms in the tissues. Eventally, bacterial counts for footpads and draining lymph nodes stabilized, with small numbers of bacilli still present in the footpads 18 months later. Persistent growth, with little sign of any immune response, was also observed in mice infected with several strains of M. avium, as well as with one strain of M. intracellulare. Other strains of M. intracellulare, as well as M. vaccae and M. nonchromogenicum, failed to establish persistent infections in normal mice, regardless of whether they were introduced by an intravenous or subcutaneous (footpad) route. The relevance of these findings is discussed in relation to antileprosy immunity in experimental animals and in humans.