Abstract: Mouse peritoneal macrophages survive a long period in vitro in a medium composed of TC199 supplemented with 40% pooled swine serum. A lag phase of multiplication of Mycobacterium lepraemurium in cell culture was observed during the first 2-4 weeks after inocelation, then followed with a logarithmic phase. The 95% confidence limit of doubling time of the organism in the latter phase is 4.9~5.5 days. The retardation and the rapidity of the growth of the organism during the two different phases are neither related to cellular acid phosphatase activity nor to the longevity of cells in culture. The growth of organisms is suppressed by the addition of rifampicin or streptomycin in culture medium.