The present invention provides highly active cyclic-di-nucleotide (CDN) immune stimulators that activate DCs via a recently discovered cytoplasmic receptor known as STING (Stimulator of Interferon Genes). In particular, the CDNs of the present invention are provided in the form of a composition comprising one or more cyclic purine dinucleotides induce human STING-dependent type I interferon production, wherein the cyclic purine dinucleotides present in the composition are 2′- or 3′-mono-fluoro substituted, or 2′3′-di-fluoro substituted mixed linkage 2′,5′-3′,5′ CDNs.