The innate immune system of mammals is a key defense mechanism against invading foreign pathogens. Innate immune stimulants may have applications as vaccine adjuvants as well as in the treatment of cancer and some viral diseases, and clinical studies have been performed using agonists of Toll-like receptors (TLRs) 7, 8, and 9. The high-throughput screens for such agonists have typically relied on the overexpression of a single TLR gene in an immortalized cell line and are inherently artificial systems that are restricted to the identification of agonists for a single receptor. The authors describe 2 assays for the identification of immunostimulants that employ primary human leukocytes cocultured with hepatitis C virus (HCV) replicon-expressing cells. In these assays, stimulation of innate immune pathways in leukocytes induces interferon (IFN) expression, which acts to inhibit HCV replication, providing a high-throughput and low-cost readout for leukocyte activation. These assays are highly sensitive and provide a physiologically relevant system for the identification of a broad range of immunostimulant agents.