OBJECTIVE The utilization of umbilical cord blood transplantation (UCBT) has been increasing because of the potential advantage of rapid accessibility and the lesser risk of graft-versus-host disease (GVHD), thus allowing less strict HLA matching. IL-28A, also known as IFN-lambda2, has been regarded as a member of a new cytokine family that shares some features with type I interferon (IFN) and was shown to have antiviral activity. The aim of this study was to identify biological activity of IL-28 on cord blood CD4(+) T cells. METHODS In this study, we cultured CD4(+) T cells with IL-28A (20 ng/ml), IL-2 (20 ng/ml) and 5microg/ml MACS Anti-Biotin MACSiBead Particles (bead-to-cell ratio 1:2) for 2 weeks. RESULTS Flow cytometry analyses showed that IL-28A cannot be effective on CD25 and Foxp3 expression on cord blood CD4(+) T cells, and it is not involved in proliferation of these cells. Treg suppression assay also showed that this cytokine cannot induce production of regulatory T cells. CONCLUSIONS We showed that IL-28A is not involved in expression of CD25 and Foxp3 markers and proliferation of CD4(+)CD25(-) T cells, and that our findings also suggest that induction of Foxp3 in T cells activated by anti-CD3/anti-CD28 does not result in the regulatory activity in these cells.