The introduction of oligodeoxynucleotides into primary cells is limited by their poor spontaneous uptake. Nevertheless, the uptake of phosphorotioate oligodeoxynucleotides (PS-ODN) by PBMCs (peripheral blood mononuclear cells) from adults has been previously reported. Not only adult diseases, but also those of newborns, are tributary of treatments with ODNs. Nevertheless, we are not aware of previous reports evaluating oligodeoxynucleotides uptake by PBMCs from newborns. We compared the uptake of a fluoresceinated PS-ODN by total PBMCs, lymphocytes and monocytes from 3 healthy newborns and 3 healthy adults before and after stimulatory conditions, by fluorescence confocal microscopy and flow cytometry. The uptake ratios, uptake kinetics and PS-ODN distributions were similar in newborn and adult PBMCs. A time-dependent PS-ODN incorporation was observed, peaking at 24 hours; evidence of three different patterns of intracellular distribution in monocytes and lymphocytes was obtained. PS-ODN uptake by monocytes was more significant than that showed by lymphocytes, and PS-ODN uptake by lymphocytes seems to be inducible by activation. The special characteristics of umbilical cord blood PBMCs such as their immunological immaturity do not preclude PS-ODN uptake. In conclusion, PBMCs from both adults and newborns would be susceptible to therapeutic approaches with PS-ODNs.