1. Analysis of a series of 135 strains of B. aertrycke indicates that at least four well-defined types may be recognised on the basis of morphology, colonial formation, virulence to mice, and to a less extent agglutination by salt, serum, and acids. These types comprise: (i) type A—the normal smooth virulent form; (ii) type B—a smooth form of low virulence; (iii) type C—a roughish, so-called ichthyotic, form of low virulence; (iv) type D—the really rough avirulent form. Between these well-defined types certain transitional forms may sometimes be recognised, the virulence of which is frequently of an order intermediate between that of the A type on the one hand, and the B, C, or D types on the other.

2. Evidence is brought to show that if a virulent strain is sub-cultured daily, under conditions known to lead to a fall in virulence of the whole culture, a gradual replacement occurs of the virulent type A bacilli by the avirulent B, C or D types. So long as an adequate proportion, which is very small, of type A bacilli persists in the culture the virulence of the whole culture remains fairly high; but when all type A bacilli have been replaced by bacilli of the B, C or D types the virulence of the whole culture falls considerably.