http://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/Griffth-ext.html

Oswald Avery expanded on Griffith's discovery of transformation and tested each of the four different macromolecules: carbohydrates, lipids, proteins, and nucleic acids. By the process of elimination, he found nucleic acids to be the "factor" that transformed non-disease causing bacteria into disease causing bacteria.  He narrowed his discovery down to the nucleic acid now known as DNA.

Textbook Link: Go to page 289; DNA 12-1