They (worked on the bio-chemical nature of transforming principle in Griffith's experiment) purified proteins DNA and RNA from heat killed S cells, they discovered protein digesting enzyme (protease) RNA digesting enzyme (RNese) did not affect transformation, Digestion with DNase inhibited transformation, concluded DNA is the heredity material.

Detailed Answer :

Oswald Avery, Colin Macleod and McCarty worked to determine the biochemical nature of 'transforming principle' in Griffith's experiment. They purified biochemicals (proteins, DNA, RNA etc.) from the heat killed S cells to see,which ones could transform live R cells into S cells.

They discovered that :

(i) Digestion of protein and RNA (using Proteases and RNases) did not affect transformation. So the transforming substance was not a protein or RNA.

(ii) Digestion of RNA with DNase inhibited transformation. It means that DNA caused transformation of R cells to S cells, i.e. DNA was the transforming substance. Therefore they concluded that DNA is the hereditary material.