Activation of human toll-like receptor-8 (TLR8) evokes a distinct cytokine profile favoring the generation of Type 1 helper T cells. A multiplexed high-throughput screen had led to the identification of N4-butyl-5-iodo-6-methylpyrimidine-2,4-diamine as a pure TLR8 agonist, and a detailed structure-activity relationship study of this chemotype was undertaken. A butyl substituent at N4 was optimal, and replacement of the 5-iodo group with chloro, bromo, or fluoro groups led to losses in potency, as did the introduction of aromatic bulk. Drawing from our previous structure-based design, several 5-alkylamino derivatives were evaluated. Significant enhancement of potency was achieved in 5-(4-aminobutyl)-N4-butyl-6-methylpyrimidine-2,4-diamine. This compound potently induced Th1-biasing IFN-γ and IL-12 in human blood, but lower levels of the proinflammatory cytokines IL-1β, IL-6, and IL-8. These results suggest that the inflammatory and reactogenic propensities of this compound could be considerably more favorable than other TLR8 agonists under evaluation.