In 1928, he conducted a series of experiments with two strains of *Streptococcus pneumoniae*, one virulent (smooth, or S) and one nonvirulent (rough, or R). He found that:
* S strain: Killed mice.
* R strain: Did not kill mice.
* Heat-killed S strain: Did not kill mice.
* Heat-killed S strain mixed with live R strain: Killed mice.
This experiment demonstrated that something from the heat-killed S strain was transforming the live R strain into a virulent form. This "transforming principle" was later discovered to be DNA.
Griffith's work was crucial in the development of our understanding of genetics and heredity, paving the way for later discoveries about DNA and its role in inheritance.
