A fully viable inoculum of BCG caused blast transformation and cell proliferation in regional lymph nodes without producing any histologic sign of an antibody response. Rates of cell division, measured as incorporation of tritiated thymidine into DNA, were proportional to the log-dose of viable BCG. Dead organisms and BCG cell walls were immunologically inert but provoked responses when suspended in oil. Cell proliferation in regional nodes increased for about 14 days and remained elevated for 28 days or more. Mycobacterial species of increasing virulence caused cellular responses of increasing intensity, implying that immunogenicity varies with virulence. By this criterion, BCG strains differ in “virulence” and show corresponding differences in immunogenicity. The Pasteur strain was the most and Glaxo the least immunogenic of the BCG strains tested. Lyophilized commercial vaccines had low viabilities and a high content of soluble antigenic material that provoked brief but intense cellular responses with features characteristic of an antibody formation. The intact BCG in lyophilized vaccines provoked smaller responses than did log-phase cultures. The latter, though inconvenient to store and distribute, gave predictable responses after prolonged storage at −70° C.