【Ａｂｓｔｒａｃｔ】　Ｏｂｊｅｃｔｉｖｅ　To investigate the change of expression on B1 cells in different sites at several times in sepsis and to explore the role of B1 cells in sepsis.　Methods　Blood samples were collected from sepsis patients and nomal persons， C57BL/6 mice were randomly divided into Sham group and cecal ligation and puncture（CLP） group， accroding to the times of puncture， CLP group was further divided to CLP-1 group and CLP-2 group， as puncture once and puncture twice respectively. Blood sample， spleen， and peritoneal lavage fluids were collected. The number and the percentage of B cells and B1 cells as well as the change of IgM which was secreted by B1 cells in abdominal cavity were measured by flow cytometry.　Results　① The results of flow cytometry revealed that the percentage of B cells and B1 cells in the blood of patients in sepsis is （9±4）% and （18±8）%， relatively lower than healthy volunteers （16±6）% vs （36±5）%（P<0.05）. ② In mice， the number of B cells and B1 cells in blood of CLP?蛳1 group is （6.00±4.20）×102/ml and （3.86±2.88）×102/ml， significant lower than Sham group （3.29±0.39）×103/ml vs （2.04±0.28）×103/ml（P<0.05）. ③ The number of B cells and B1 cells in spleen of CLP-1 group is （9.13±3.24）×105/ml and （2.08±0.71）×105/ml， significant lower than Sham group （6.71±0.71）×106/ml vs （1.29±0.88）×106/ml（P<0.05）. ④ As to Sham group， the percentage of B cells in peritoneal cavity of CLP-1 group after CLP 24 h and 48 h in mice is decreased（P<0.05）. However， only at the time of 6 h and 24 h， percentage of B1 cells is declined（P<0.05）. ⑤ The percentage of secretion of IgM in B1 cells is （61±7）% when mice experienced severe sepsis in CLP-2 group， which is lower than Sham group（84±6）%（P<0.05）.　Conclusions　Both the number and the function of B1 cells have negative changes， it's suggested that B1 cells may involved in immune dysfunction of sepsis.