Since Mycobacterium lepraemurium (Mlm) is closely related to Mycobacterium intracellulare, with respect to its antigenicity, attenuated Mlm obtained by passages on 1% Ogawa egg medium was examined for possible effects as a vaccine against M. intracelJulare infection. Four weeks after subcutaneous injection of Mlm (1.4 x 107 bacilli), mice were infected intravenously with 2.4 x 107 organisms per mouse of M. intracellulare. During the first 12 weeks, no difference was noted in the rate of progression of M. intracellulare infection between mice given or not given Mlm-vaccine, when severity of the infection was measured on the basis of the number of viable M. intracellulare in the lungs and spleen. Therefore, Mlm-vaccine is probably not efficacious in protecting mice against M. intracellulare infection.