Cyclic GMP-AMP synthase (cGAS) is a pattern recognition receptor (PRR) that senses double stranded DNA (dsDNA) in the cytosol and this leads to the activation of stimulator of interferon genes (STING) via the secondary messenger 2’3’-cyclic GMP-AMP (2’3’-cGAMP). STING then recruits TANK binding kinase 1 (TBK-1) and this complex can phosphorylate and activate interferon regulatory factor 3 (IRF3) leading to the induction of type I interferons and other antiviral genes. The cGAS:DNA complex catalyzes the synthesis of 2’3’-cGAMP and the purpose of the protocol presented here is to measure the in vitro activity of purified cGAS in the presence of dsDNA. The protocol was developed to elucidate the relationship between dsDNA length and the level of cGAS activity. The method involves an in vitro reaction with low concentrations of cGAS and dsDNA followed by quantification of the reaction product using anion exchange chromatography. The low concentrations of cGAS and dsDNA and the high sensitivity of this assay is a key advantage when comparing different DNA fragments’ ability to activate cGAS.