At present , cadmium2induced damage to fish immune system especially immune cells is hardly seen in references. In the paper , the auther studied for the first time the cadmiumly2induced apoptosis to peripheral blood mononuclear cell (PBMC) . In the paper , the auther studued for the first time the cadmiumly2induced apoptosis to peripheral blood mononuclear cell (PBMC) of crussian carp Carassius auratus (mainly T cell ,βcell and monocyte) by electron microscopy , single cell gel electrophoresis (SCGE) , DNA gel electrophoresis and flow cytometry (FCM) . At the beginning of our procedure , fish sample (400g even purchased from Suzhou Xiangcheng District aquafarm were randomly assigned to glass aquaria ( 100cm ?60cm ?50cm) at five of each. All the culture conditions were kept for 4 weeks including continuously aeration , twice diets daily (08 :00 and 14 :00) at the rate of 3 % body weight , half to two2thirds of daily water exchange , 23 to 27 ℃temperature range , and over 5mg?L -1dissolved oxygen content. Then , the test fish were intraperitonealy injected with Cd2 at 1. 25mg?kg 1 , and the control is Cd2 free. After injection on the 14th , 7th , 21st day , 5mL blood samples (within 150IU?mL -1 heparin) from the fish tail vein were mixed with equal volume PBS (0. 01mol?L-1, pH7. 2) , these duluted blood was gently covered by 5ml Ficoll2Paque , the band containing mainly PBMC between plasma and isolation solution was observed after being contrifuged at 1500r?min 1 for 10 min , then cellected the PBMC from this band , and washed twice in PBS , counted and adjusted to a density of 1 ?107 ind ?mL -1 in RPMI21640 plus 10 % FBS. Under electron microscope, the treated cells appeared chromatin marginating , condensating and extruding into cytoplasm , unclear structure of organelles and large vacuoles in cytoplasm , while the normal did evenly2distributed chromatin in U shape nuclei and intact organelles . By SCGE , the treated cells exhibited comet head of DNA margination and comet tails of DNA migration , while the normal did full nuclear skeleton ; the apoptotic rate was calculated according to the fraction of the apoptotic cells to the total cells at 14 , 17 and 21d were 19. 4 % , 16. 4 % , 5. 6 % respectively , which were significantly higher than that of the normal (1. 1 %) ( P < 0. 05) . By DNA gel electrophoresis , the treated cells had typical nucleosome bands (180 -220bp integer fold) 14 , 17 and 21d respectively , and these ladder bands at 14d and 17d were more apparent than those of 21d , while the normal had only an DNA band. In FCM , the treated cells showed aneuploid peaks , while the normal didn’t ; the nomber of cells in aneuploid peak were calculated aicording to the fraction of cells in aneuploid peak to the total cells at 14 , 17 and 21d were 17. 5 % , 23. 6 % and 8. 2 % respectively , which were significantly higher than that of the normal (1. 4 %) ( P < 0. 05) . The above evidences was suggested that cadmium could induce apoptosis of PBMC at this dose in vivo , and its apoptosis rate could reach summit valve in 14 -17d and then drop at 21d. So apoptosis of lower dose cadmium2stressed PBMC is likely to play a key role in the damage of cadmium to the immune system of C. auratus.