Compositions and methods relating to immunostimulatory RNA oligomers are provided. The immunostimulatory RNA molecules are believed to represent natural ligands of one or more Toll-like receptors, including Toll-like receptor 7 (TLR7) and Toll-like receptor 8 (TLR8). The compositions and methods are useful for stimulating immune activation. Methods useful for screening candidate immunostimulatory compounds are also provided.