The study was to investigate the proliferation and apoptosis effect of nociceptin/orphanin FQ (OFQ) on 562 cells in vitro. Inhibition of K562 cells proliferation was measured by MTT assay. Morphological assessment of apoptosis was performed with Wright staining and transmission electron microscope. The apoptosis peak was measured by flow cytometry. DNA fragmentation was visualized by agarose gel electrophoresis. The results showed that OFQ time-dependently and no-dose-dependently inhibited the proliferation of K562 cells at concentrations of 10(-6) - 10(-13) mol/L. Discrete maximum of cytolytic activity was detected at concentrations of 10(-6) - 10(-7), 10(-9), 10(-12) mol/L. Compared with the control group K562 cells, the cells treated with OFQ at concentration of 10(-9) mol/L for 72 hours showed typical characteristics of apoptosis under transmission electron microscope. Apoptosis peak was found by FCM at concentration of 0, 10(-7), 10(-8), 10(-9) mol/L of OFQ for 72 hours, apoptosis rates were 0%, 22.8%, 23.8% and 26.5% respectively. DNA agarose gel electrophoresis revealed nuclear fragmentation (DNA ladder). It is concluded that OFQ can inhibit the proliferation of K562 cells and induce the apoptosis in K562 cells.