Using a cell sorter, CD16-CD56bright natural killer (NK) cells were sorted from decidual mononuclear cells at an early stage of pregnancy. These cells were examined by the reverse transcriptase-polymerase chain reaction (RT-PCR) method for their expression of mRNA coding for the following 12 cytokines: IL-1 beta, IL-2, IL-3, IL-4, IL-5, IL-6, granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), macrophage colony-stimulating factor (M-CSF), tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), and leukemia inhibitory factor (LIF). Although mRNA coding for every cytokine was detected in decidual mononuclear cells, mRNAs coding for only G-CSF, GM-CSF, M-CSF, TNF-alpha, IFN-gamma, and LIF were detected in CD16-CD56bright NK cells. Also, the supernatant of CD16-CD56bright NK cell cultures was found to contain G-CSF, GM-CSF, M-CSF, TNF-alpha, IFN-gamma, and LIF. These findings indicate that CD16-CD56bright NK cells produce many different cytokines and that these cytokines may play an important role in a successful pregnancy.