Conversely DNA can be labelled with P³², which will not be incorporated into proteins because proteins do not contain phosphorus. Once labelled, samples of DNA and protein can be distinguished from each other because the two radioisotopes, P³² and S³⁵ emit radiation of different characteristic energy.

Hershey and Chase prepared a radioactive sample of T2 bacteriophage one in which protein was labelled with S³⁵ and other in which DNA was labelled with P³², by infecting T2 phage on E.coli bacterial lawn grown with S³⁵ and P³²-labelled nutrients . The labelled phages produced by this culture were used to infect the next, non radioactive culture of E coli. However, this time the infection procedure was interrupted a few minutes after inoculation by agitating the cells in a waring blender. These few minutes were long enough for the phage genes to enter the bacteria, but not enough for new bacteriophages to be synthesized to kill the bacteria. The culture was then centrifuged so that the relatively heavy bacterial cells containing the phage genes collected at the bottom of the tube, leaving the empty phage particles in suspension. Hershey and Chase discovered that over 80% of the P³² was present in the bacterial pellet. The bacteria were than allowed to continue through the infection process and produce new phages. Almost half of the P³², but less than 1% of the S³², was present in these new phage particles. Clearly, this experiment has proved that the genetic material was DNA and not protein.