1. A variant intermediate between the classical R and S forms has been isolated by selective procedures from a rough strain of pneumococcus originally derived from Type II S.
2. The intermediate variant D39/Int53 is avirulent for mice, forms rough colonies, and does not possess a demonstrable capsule. However, it synthesizes SSSII which is immunologically indistinguishable from that produced by fully encapsulated pneumococcus Type II, though in much smaller amount. The polysaccharide is present as a surface component and as it exists in the cell is highly antigenic for rabbits.
3. An extract of the intermediate variant causes the transformation in vitro of an R strain into a variant resembling the intermediate in SSSII production but without any apparent alteration in the colonial characteristics of the R variant.
4. The intermediate variant is convertible in vivo, into a fully encapsulated strain of pneumococcus Type II. Transformation of the intermediate to a heterologous type of pneumococcus (Type III) was unsuccessful.
5. A method is described for the preparation of transforming extracts of pneumococci utilizing the massive growth of the organisms obtained in the presence of a large concentration of glucose.