We have shown previously that polynucleotides enhance in vitro antibody and Ig production in response to T-dependent antigens in mice and augment Ig production by adult human peripheral blood mononuclear cells. Herein, we report their effects on umbilical cord blood mononuclear cells (CBMNC) obtained from full-term babies. CBMNC produced much less IgM/IgG and an almost negligible amount of IgA in response to various stimuli compared with adult peripheral blood mononuclear cells. The supplementation of yeast RNA augmented spontaneous and T-dependent IgM (p < 0.01) but not IgG production by CBMNC. This action was largely attributable to polynucleotides, which appeared to exert their actions in a dose-dependent manner at the initial stages of culture. Their actions were dependent upon the presence of T cells, but they also enhanced spontaneous IgM production by CBMNC in the absence of T cells. Preincubation of T cells from CBMNC and peripheral blood mononuclear cells with RNA for 3 h before the culture resulted in enhanced IgM production, independent of the stimulants used. Thus, polynucleotides appear to exert actions on immature human T cells as well as other lineage cells in vitro. Their actions may be dependent on the presence or absence of antigens or other stimuli and the nature of the stimuli (T dependent versus T independent). These findings may further support the potential importance of nucleotides contained in human breast milk.