Aquired resistance against tuberculosis depends on the survival, multiplication and persistence of BCG in the host organs. Therefore, the viability and stabilisation during storage of BCG vaccine constitute a major attribute for good immunogenicity. Different factors which influence BCG viability have been studied. Among the more important ones it was found that a very large number of viable bacilli are killed when vaccine is manufactured in the conventional way by ball milling the surface-grown bacillary mass. The dispersed deep-grow BCG offers the advantage that all bacilli are live in the fresh suspension before stabilisation procedures are applied. Two procedures of stabilisation have been used, freezing and freeze-drying. Preservation by resuspending BCG in different cryoprotective solutions has been followed by storage at low temperatures. Complete survival is obtained when this vaccine was frozen in glycerol solutions and stored for many years at -70 degrees C. Freeze-drying killed more than 50% of the live bacilli in the fresh suspension; the remaining freeze-dried live bacilli were preserved at -30 degrees C for 20 years or at 4 degrees C for at least one year. They also resist exposure to 37 degrees C for one month. Nevertheless, most of the BCG bacilli in freeze-dried vaccines are dead. In conclusion the best BCG viability is obtained with young dispersed-grown bacilli by both freezing and freeze-drying. Freezing is a good method of stabilisation for research purposes or cancer immunotherapy, and freeze-drying for BCG vaccination campaings in tuberculosis prevention. However, BCG stabilisation still needs improvement.