Abstract： Objective: To observe the regularity of change in high mobility group protein box 1 (HMGB1) content in serum and spleen of mice with multiple organ dysfunction syndrome (MODS), to analyze the correlation between HMGB1 content and major histocompatibility complex (MHC)-Ⅱ-I-Ab expression on monocytes in blood and spleen, and to explore the effect of HMGB1 on immune function of circulating monocytes and splenocytes. Methods: One hundred 8-week-old male 57BL/6 mice were randomly divided into normal group and experimental group subdivided into 8 subgroups: 3, 8, 12 hours, 1, 2, 3, 5-7 days and 10-12 days post zymosan injection (PZI). MODS model was replicated by injecting zymosan into the peritoneal cavity. At each time point, blood and spleen were collected to detect HMGB1 content and the rate of I-Ab positive monocytes. Results: In normal and PZI 3-hour, 8-hour mice, serum HMGB1 was not detected, but it significantly increased at PZI 12 hours. In spleen of normal mice, there was low level of HMGB1 expression. In zymosan-treated mice, HMGB1 started to rise in spleen at PZI 3 hours. Subsequently, HMGB1 content in both serum and spleen significantly increased, and it reached the peak level in 1-2 days, decreased in 5 days, and then increased in 10-12 days. The number of I-Ab positive monocytes in circulating blood and spleen decreased at 1-2 days (t=9.589, 4.432, P<0.01) and 10-12 days following the challenge, forming a two trough like decrease, just corresponding with two-peak increase of HMGB1. However, at 3 hours after zyrnosan challenge, I-Ab expression on circulating monocytes was downregulated (t=5.977, P<0.01), while that in spleen upregulated (t=4.814, P<0.01). Conclusion: In mice with MODS, up-regulated HMGB1 expression can regulate I-Ab expression on monocytes to depress their ability of presenting antigen, which results in immune disturbance contributing development of MODS.