Safe and cost-effective adjuvants are a critical requirement for subunit vaccine development. We report here the in vivo activity of a series of fully synthetic LPS receptor agonists that have been shown to activate NF-kappaB signaling through the Toll-like receptor 4 (TLR4). These compounds boost antibody responses to protein antigens when coadministered at microgram doses in mice. At these dosage levels no adverse effects are observed. Antibody responses are largely IgG1, with enhanced IgG2a, and down-regulated IgE as compared to alum adjuvanted immunization. Stimulation of Th1 is confirmed by enhanced gamma-interferon production after in vitro antigen restimulation of spleen cells from mice immunized with the synthetic adjuvants. The adjuvants are active by both subcutaneous and intranasal routes of vaccine administration, and in the latter case can amplify both serum IgG and serum and mucosal IgA responses. The compounds must be administered at the same site with antigen to boost anti-vaccine antibody. These fully synthetic ligands of the innate immune system offer the potential for use as effective, safe, and nonbiologically-derived adjuvants.