Background Newborns have frequent infections and manifest impaired vaccine responses, motivating a search for neonatal vaccine adjuvants. Alum is a neonatal adjuvant but might confer a TH 2 bias. Toll-like receptor (TLR) agonists are candidate adjuvants, but human neonatal cord blood monocytes demonstrate impaired TH 1-polarizing responses to many TLR agonists caused by plasma adenosine acting through cyclic AMP. TLR8 agonists, including imidazoquinolines (IMQs), such as the small synthetic 3M-002, induce adult-level TNF from neonatal monocytes, but the scope and mechanisms of IMQ-induced activation of neonatal monocytes and monocyte-derived dendritic cells (MoDCs) have not been reported. Objective We sought to characterize IMQ-induced activation of neonatal monocytes and MoDCs. Methods Neonatal cord and adult peripheral blood monocytes and MoDCs were cultured in autologous plasma; levels of alum- and TLR agonist–induced cytokines and costimulatory molecules were measured. TLR8 and inflammasome function were assayed by using small interfering RNA and Western blotting/caspase-1 inhibitory peptide, respectively. The ontogeny of TLR8 agonist–induced cytokine responses was defined in rhesus macaque whole blood ex vivo. Results IMQs were more potent and effective than alum at inducing TNF and IL-1β from monocytes. 3M-002 induced robust TLR pathway transcriptome activation and TH 1-polarizing cytokine production in neonatal and adult monocytes and MoDCs, signaling through TLR8 in an adenosine/cyclic AMP–refractory manner. Newborn MoDCs displayed impaired LPS/ATP-induced caspase-1–mediated IL-1β production but robust 3M-002–induced caspase-1–mediated inflammasome activation independent of exogenous ATP. TLR8 IMQs induced robust TNF and IL-1β in whole blood of rhesus macaques at birth and infancy. Conclusions IMQ TLR8 agonists engage adenosine-refractory TLR8 and inflammasome pathways to induce robust monocyte and MoDC activation and represent promising neonatal adjuvants.