Background The T-helper type 1 (Th1) trophic properties of bacterial cytosine-phosphate-guanine (CpG) motifs have made them logical adjuvants both for the suppression of Th2-mediated allergic disease in early life and for promoting vaccine responses in neonates who have relatively immature Th1 function. However, little is known about their effects on immature immune responses in this period.Objectives To compare the effects of CpG on adult and neonatal cellular immune responses to various stimuli.Methods The immune responses of mononuclear cells (MC) derived from neonates (n = 25) and their mothers (n = 25) were compared in vitro. These were stimulated with house dust mite (HDM), CpG B, CpG C, non-CpG oligodeoxynucleotides (ODN) or diphtheria toxoid (DT) in optimized conditions. In parallel cultures, CpGs were combined with HDM or DT antigens to assess the effect of the various ODN on these antigen-specific responses. Lymphoproliferation and cytokine responses IL-13, IFN-gamma, IL-6, IL-10, TNF-alpha) were measured for all of the cultures described above.Results Although neonates showed attenuated lymphoproliferation to CpG, the production of antigen-presenting cell-derived cytokines such as IL-6 and IL-10 and the up-regulation of major histocompatibility complex class II (HLA-DR) were detected at adult levels. T cell-derived cytokines (IL-13 and IFN-gamma) were not detectable in response to CpG alone. Most neonates also failed to produce detectable IFN-gamma to HDM or DT (unlike adults), but readily produced IL-13 to these stimuli. The addition of CpG resulted in an increase in neonatal IFN-gamma production in response to HDM (P = 0.011) and a similar but non-significant trend with DT. However, rather than inhibiting Th2 IL-13 responses, the addition of CpGs was associated with a significant increase in the IL-13 responses to HDM (P = 0.016) and DT (P = 0.03), effects not seen in adults.Conclusions This study provides further evidence that neonatal MC responses to CpG are functionally different from adults, and do not show clear Th1 polarization. The CpG associated increase in Th2 responses may reflect a potentiation of the normal neonatal Th2 propensity, or non-specific activation of neonatal MC.