OBJECTIVE To study the toxicity of zinc oxide nanoparticles (ZnO-NPs) on murine macrophage Ana-1 cells and the mechanism.METHODS Ana-1 cells were incubated with ZnO-NP (2.5-160 mg· L-1).Cell viability was investigated by MTT assay.The integrity of cell membrane was investigated by acridine orange-ethidium bromide (AO-EB) staining.The intracellular uptake of ZnO-NP and the percentage of sub-G1 of Ana-1 cells were detected by flow cytometry.Zinc ions were determined by fluorescent probe.The change of cell viability was studied after chelating zinc ions with ethylene diamine tetraacetic acid (EDTA).RESULTS ZnO-NP 2.5,5,10 and 20 mg· L-1 decreased cell viability of Ana-1 cells (r=0.905,P＜0.05) in a concentration-dependent manner.The cell viability was decreased to 27.9％ after exposure to ZnO-NP 20 mg· L-1.Intracellular uptake of ZnO-NP was increased after Ana-1 cell incubated with ZnO-NP at concentrations ranging from 40 to 160 mg· L-1 (P＜0.05).There were obvious free zinc ions in the cells.EDTA 2.5 mmol· L-1 significantly increased the cell viability decreased by ZnO-NP 20 mg· L-1 (P＜0.05).Chelating free zinc ions significantly mitigated ZnO-NP induced cell toxicity (P＜0.05).CONCLUSION Cytotoxicity and apoptosis of Ana-1 cells induced by ZnO-NP might be related to intracellular uptake of ZnO-NP and release of zinc ions.