They performed experiments on E.coli to prove that DNA replication is semi-conservative. They first grew the bacteria in a medium containing ${}_{15}\mathrm{NH}_4\mathrm{Cl}$ (in which ${}_{15}\mathrm{N}$ is the heavy isotope of nitrogen) for many generations.

Then they transferred the cells into a medium with normal ${}_{15}\mathrm{NH}_4\mathrm{Cl}$ (in which ${}_{14}\mathrm{N}$ is the lighter isotope) and took the samples at various definite time intervals as the cells multiplied. The extracted DNAs were centrifuged and measured to get their densities.

The DNA extracted from the culture after one generation of transfer fro then ${}_{15}\mathrm{N}$ medium to ${}_{14}\mathrm{N}$ medium, (i.e., after 20 minutes E.coli divides every 20 minutes) showed an intermediate hybrid density, i.e., both heavy and light nitrogen, which proved the semi-conservative nature of DNA.