Hershey and Chase used bacteriophage, called T2. It was an experimental tool they used to prove that DNA was the genetic material. In their experiments, Hershey and Chase showed that when bacteriophages, which are composed of DNA and protein, infect bacteria, their DNA enters the host bacterial cell, but most of their protein does not.
Their entire experiment was based on the ability of T2 phage to infect E. coli cells, hence they radioactively tagged DNA of one colony and the other colony protein was radioactively tagged,