We have shown previously that pneumococcal polysaccharides behave as human T cell-independent type 2 Ag. When cultured in vitro with type 4 pneumococcal polysaccharides (PS4), human neonatal B cells do not or only marginally respond. Limiting dilution analysis of neonatal B cells polyclonally activated by a combination of phorbol esters, calcium ionophore, and T cells and T cell factors, however, showed that Ag-reactive B cells are present in cord blood. The frequency of anti-PS4 reactive B cells in cord blood is comparable with that of adult peripheral blood. In order to obtain more insight into the activation requirements of these PS4-reactive neonatal B cells, 8-mercaptoguanosine (8MGuo) was added to in vitro cultures. Addition of 0.5 to 1.0 mM 8MGuo resulted in a 3- to 10-fold amplification of the anti-PS4 response. the effect of 8 MGuo was most prominent when added 3 days after initiation of the culture. Based on kinetic studies, we propose that in vitro activated cells are target for 8MGuo. These data further indicate that neonatal unresponsiveness to polysaccharide Ag is not due to the physical absence of Ag-reactive B cells but more likely to be the consequence of different activation requirements as compared with adult B cells.