Two major facts were already known about DNA. The first was that it is contained in the nucleus of every living cell. The second was that, in addition to sugar and phosphate, DNA was composed of two bases: pyrimidines, of which there are two types (cytosine and thymine), and purines, of which there are also two types(adenine and guanine).
He discovered that adenine and thymine existed in equal proportions in all organisms, aswell as cytosine and guanine, and that the proportions between the two pairs were different depending on the organism.
From his research Chargaff drew the conclusion that it was the DNA in the nucleus of the cell that that carried genetic information rather than the protein.