Griffith's experiment with Pneumococcus proves that DNA is the transforming principle. Griffith performed the experiment in mice by using two strians of Pneumococcus pneumonia, the rough (R) and the smooth (S) strian. The S strain bacteria have a mucous (polysaccharide) coat, while R strain does not.

Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia. Griffith was able to kill bacteria by heating them. He observed that heat-killed S strain bacteria injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice. He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria. Some ‘transforming principle’, transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent.