Among the various parameters which may contribute to Mycobacterium bovis BCG vaccination efficiency, the choice of the vaccine strain may play an important role. In the present study, we therefore compared the immunogenicity of five different BCG strains that are commonly used for BCG vaccine production (Glaxo 1077, Japanese 172, Pasteur 1173P2, Prague, and Russian strains). The comparison of the growth capacity of these BCG strains in BALB/c and C3H mice demonstrated that a great difference exists between the capacity of various BCG strains to multiply and persist in target organs. A much lower recovery of BCG could be shown in mice immunized with Prague and Japanese BCG strains. T-cell responses of BCG-immunized mice were also examined by analyzing T-cell proliferative responses, cytokine production, delayed-type hypersensitivity responses, and cytotoxic activity. All these assays demonstrated that BCG immunization induced strong CD4+ T-cell responses, mostly of the Th1 type, as demonstrated by interleukin-2 and gamma interferon production. These studies also demonstrated that there are differences between BCG strains in stimulating these T-cell responses. A lack of induction of cytotoxic activity was observed following immunization with the Japanese strain. Lower anti-purified protein derivative antibody responses were also observed after intravenous or oral immunization with this BCG strain. Finally, the protective activity of these BCG strains was tested by measuring the capacity of immunized mice to eliminate recombinant Pasteur and Japanese BCG strains which expressed beta-galactosidase. The results of these experiments clearly demonstrated that the Prague and Japanese strains were unable to protect mice against a second mycobacterial challenge whereas mice immunized with the Glaxo, Pasteur, or Russian strain eliminated the recombinant BCG very efficiently. Altogether, the results of the present study strongly support the view that there are considerable differences in the immunogenicity of various BCG vaccine strains and that these differences may play a major role in BCG vaccination efficiency.