When Frederick Griffith injected a mixture of heat killed S(virulent strain) and R(non virulent strain) bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice.
He concluded that the R strain bacteria had 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 synthesis a smooth polysaccharide coat and become virulent.
This is actually, due to the transfer of the genetic material (the transforming principle).