Objective To investigate the difference of the protein expression in serum of rats with acute edematous pancreatitis (AEP) and acute necrotizing pancreatitis (ANP),and to investigate serum marker for acute pancreatitis severity.Methods The model of AEP and ANP was induced by retrograde injection of 4％ or 5％ sodium taurocholate into the biliopancreatic duct.Weak cation exchange (WCX2) and surface enhanced laser desorption/ionization time of flight mass spectrometry ( SELDI TOF MS) was used to investigate the difference of the protein expression in serum of rats with AEP and ANP.Results Thirty-eight spectral peak clusters which had significantly different signal intensities between AEP and ANP sera at mass charge ratio between 1000-50 000(P ＜0.05) were detected.The peak clusters at 9500 and 9700 in the sera of AEP were higher than that in ANP rata.Conclusions Serum analysis with SELDI-TOF MS can detect the difference of the protein expression in rats with AEP and ANP.The decreased expression of the protein of molecular weight of 9500 and 9700 may be a signal of AEP transition into ANP.