WHO oversees the quality control of BCG vaccine via a system that includes regular testing of products by in vitro methods and clinical trials. Three parent strains of BCG (Glaxo-1077, Tokyo-172, and Pasteur-1173P2) account for over 90% of the vaccines currently in use worldwide. Important characteristics of the vaccine preparations are summarized here, along with their physical-chemical properties. In instances where diagnostic criteria for tuberculosis are stringent, there is no evidence that when administered to newborns different preparations of BCG vaccine exhibit different efficacies; however, the incidence of BCG-associated adverse reactions does correlate with the type of preparation. Other factors, including dose, administration technique, and recipient characteristics are also important in determining vaccine-associated reactions.