The experiment you're describing is most likely referring to the work of Frederick Griffith in 1928. He conducted experiments with two strains of *Streptococcus pneumoniae* bacteria:

* Smooth (S) strain: This strain had a smooth capsule and was virulent, meaning it could cause pneumonia in mice.

* Rough (R) strain: This strain lacked the capsule and was non-virulent.

Griffith's experiment:

1. Injection of S strain: Mice injected with the S strain died.

2. Injection of R strain: Mice injected with the R strain lived.

3. Injection of heat-killed S strain: Mice injected with heat-killed S strain lived.

4. Injection of a mixture of heat-killed S strain and live R strain: Mice injected with this mixture died.

The surprising finding: When Griffith analyzed the dead mice from the fourth group, he found live S strain bacteria. This led him to the conclusion that something from the heat-killed S strain had transformed the live R strain into the virulent S strain.

This experiment is crucial because it provided the first evidence that genetic material could be transferred between bacteria, a process now known as transformation. Although Griffith didn't know what this "transforming principle" was, his work paved the way for later discoveries by Avery, MacLeod, and McCarty, who identified DNA as the genetic material responsible for transformation.