The First demonstration of bacterial transformation.
Experiments done by Frederick Griffith (in London) in 1928 found there were two different types of the bacterium Streptococcus pneumoniae:

An "S" or SMOOTH coat strain, which is lethal to mice.
Figure 11_1a from Griffiths et al., 1996
An "R" or rough strain, which will not hurt the mouse.
Figure 11_1b from Griffiths et al., 1996
Griffith found that he could heat inactivate the smooth strain.
Figure 11_1c from Griffiths et al., 1996
However, if he were to take a mixture of the heat-inactivated S strain,
mixed with the R strain, the bacteria would die.  Thus there was some
Material in the heat-killed S strain that was responsible for "transforming"
the R strain into a lethal form.
Figure 11_1d from Griffiths et al., 1996
 
Fred Griffith (and a lab co-worker) was killed in their laboratory in 1940 from a German bomb.  However, their work continued on in the U.S., and in 1944, Oswald Avery, C.M. MacLeod, and M. McCarty carefully demonstrated that the ONLY material that was responsible for the transformation was DNA - thus, DNA was the "Genetic material" - however, many scientists were still not sure that it was REALLY DNA (and not proteins) that was the genetic material.

In 1952, Alfred Hershey and Martha Chase (she was an UNDERGRADUATE at the time!) demonstrated clearly that DNA must be the genetic material, using bacteriophage T2.