A phage is a small virus that infects bacteria. Like a virus, it consists of a protein hull that encloses the genetic material. When a phage infects a bacterium, it inserts its genetic material into the bacterium, while its hull remains outside.
In a first experiment, T2 phages with radioactive 32P-labeled DNA infected bacteria. In a second experiment, T2 phages with radioactive 35S-labeled protein infected bacteria. In both experiments, bacteria and phage remains were then separated by centrifuge. In the first experiment, most of the used radioactive molecules were found in the infected bacteria, while in the second experiment most radioactivity was found in the phage remains. That shows the DNA to be the genetic material used to infect and the protein carrying no genetic information.