This is more formally called the Avery-MacLeod-McCarty experiment. This experiment was done to demonstrate that bacterial transformation is caused by DNA. In Griffith's experiment, a rough strain of S. pneumoniae which is non-virulent which did not kill the mouse. A smooth strain of S. pneumoniae which is virulent killed the mouse. A heat-killed smooth strain of S. pneumoniae which was assumed to be non-virulent did not kill the mouse. However, the combination of a rough strain and a heat-killed smooth strain killed the mouse. This proves that the DNA from the heat-killed smooth strain lead to bacterial transformation of the rough strain into the smooth strain, therefore killing the mouse.