Abstract: In order to investigate the effects of 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone(NNK) on the immunity of mice,the mice were randomly divided into five groups:NNK low-dose,middle-dose and high-dose groups(150,300,600 μmol/kg),normal control group,and positive control group(cyclophosphamide,200 mg/kg). Seven days after intraperitoneal injection,the routine blood tests of the mice were carried out,their thymuses and spleens were taken out and weighed,and thymus index and spleen index were calculated,then the phagocytosis of macrophages in abdominal cavity of mice was determined with fluorescence microscope counting method,the function of plaque forming cells(PFC) was detected with quantitative hemolysis spectrophotometry,delayed type hypersensitivity(DTH) was tested with foot palm thickening method,and T lymphocyte subgroups was determined by a flow cytometry.The results showed that:Comparing with the normal control group,1) DTH reduced by 28.97% in the mice of NNK low-dose group(P<0.05);2) In NNK middle-dose and high-dose groups,platelet counts,thymus weight,thymus index,DTH,PFC,the percentage of macrophages phagocytosis,phagocytosis index reduced by 17.50%-18.15%,44.82%-50.00%,44.50%-44.93%, 38.91%-55.79%,18.84%-27.53%,23.79%-29.75% and 26.35%-28.14%(P<0.05),respectively;3) The spleen weight and percentage of CD3⁺CD8⁺T lymphocyte decreased by 19.67% and 23.63% in NNK high-dose group(P<0.05).