The protective effect in bank voles of some strains of BCG.

The bank vole, like the guinea-pig, can be immunized against tuberculosis with very small doses of BCG, presumably because BCG is sufficiently virulent for both these species to multiply freely until the immunization of the animal has reached a maximum. Large doses of BCG induce an earlier, but not an ultimately stronger, immunity. However, the smallest immunizing dose is not the same for all BCG strains and this is a feature that may be used to differentiate strains. In the studies reported by the authors, the smallest immunizing dose for the bank vole was estimated for 11 BCG strains, some of them widely used in man. The results indicate potency ratios of 20: 1 between the strongest and the weakest strains. The ranking of strains in terms of the smallest immunizing dose was found to be correlated in most, but not in all, cases with a previously reported ranking of the lethal effects of the various strains in golden hamsters. The implications of these findings for production requirements and for reference preparations of BCG are discussed.