Programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) play a major role in suppressing the immune system during the formation of the PD-1/PD-L1 pathway, which transmits an inhibitory signal to reduce T cell activity. PD-L1 is often expressed in various malignant tumors. In contrast, PD-1 is generally observed in activated lymphocytes and myeloid-derived dendritic cells. Of the malignant cells, only Jurkat cells under special conditions and angioimmunoblastic T-cell lymphoma tissue cells express PD-1 on their surface.To clarify whether the PD-1/PD-L1 pathway participates in the immunotolerance of small-cell lung cancer (SCLC) cells, we examined the expressions of PD-1 and PD-L1 on the cell surface of SCLC cell lines using flow cytometry and reverse transcription polymerase chain reaction.Among the four SCLC cell lines examined, only SBC-3 expressed both PD-1 and PD-L1.We demonstrated that both PD-1 and PD-L1 molecules were co-expressed on the surface of SCLC cells. Although the biological implications of this remain unclear, we speculate that PD-1 and its ligand on the SCLC cells may participate in the growth inhibition of tumor cells as reported in cytotoxic T cells.