Four BCG strains, French 1173-P2, Danish 1331, Japanese 172 and Glaxo-1077, among those most widely used throughout the world for tuberculosis vaccination, were prepared according to the respective conditions in each BCG-production unit. These 16 vaccine preparations were distributed and compared using standardized in vitro and in vivo conditions, based on optical density and bacillary mass/ml. The results presented in this report concern only the 4 freeze-dried BCG strains prepared by the Institut Pasteur BCG unit. It is shown that these 4 preparations differ in their in vitro characteristics such as growth, morphology, heat stability and viability, tested by colony-forming units and the ATP content. In addition, statistically significant differences were observed in biological control tests performed in guinea-pigs and mice when they were immunized with the same concentrations of 4 reconstituted vaccines. For instance, in tests measuring hallmarks of cell-mediated immunity--such as local granuloma formation, spleen index, delayed-type hypersensitivity to tuberculin, and non-specific immunopotentiation to unrelated antigen--a ranking order of relative immunopotency of the four strains was made, giving the following classification in decreasing order: French 1173-P2, Danish 1331, Japanese 172 and Glaxo-1077. Specific acquired resistance after BCG vaccination in mice was obtained with all four vaccines. However, higher protection was observed in mice vaccinated with Glaxo-1077 and French 1173-P2 strains. No obvious correlation existed between the intensity of cellular post-vaccination resistance and the above CMI tests, except those concerning the three-week index of BCG infection measured in the spleen, which correlated with the protection test.