Viable suspensions of BCG, an attenuated strain of Mycobacterium bovis, have been previously shown to immunize mice against infections with M. leprae. Usually, the mice have been vaccinated about 1 month before challenge. Experiments have now been carried out with single intradermal injections of BCG given before or after the M. leprae challenge. Approximately equal immunizing effect was seen in one experiment when the BCG was given at -168, -119, -70, and -28 days relative to challenge. Approximately equal protection was observed in another experiment when the vaccine was given at -28, +28, and +56 days. In the latter experiment, however, vaccine given at +91 days appeared to be somewhat less effective. Enlargement of the lymph nodes regional to the intradermal vaccine site persisted for at least the duration of the experiment, approximately 400 days. Thus, antigenic stimulation appears to have continued throughout the period of observation.