We describe novel STING-activating cyclic dinucleotides whose constituent nucleosides are adenosine and inosine and that vary by ribose substitution, internucleotide linkage position, and phosphate modification. In mammalian cells in vitro, some of these cAIMP analogs induce greater STING-dependent IRF and NF-κB pathway signaling than do the reference agonists for murine (DMXAA) or human (2',3'-cGAMP) STING. In human blood ex vivo, they induce type I interferons (IFNs) and proinflammatory cytokines: for the former, 3',3'-cAIMP (9; EC50 of 6.4 μM) and analogs 52-56 (EC50 of 0.4-4.7 μM), which contain one or two 2'-fluoro-2'-deoxyriboses and/or bis-phosphorothioate linkages, are more potent than 2',3'-cGAMP (EC50 of 19.6 μM). Interestingly, 9 induces type I IFNs more strongly than do its linkage isomers 2',3'-cAIMP (10), 3',2'-cAIMP (23), and 2',2'-cAIMP (27). Lastly, some of the cAIMP analogs are more resistant than 2',3'-cGAMP to enzymatic cleavage in vitro. We hope to exploit our findings to develop STING-targeted immunotherapies.