In previous experiments reared fish were submitted to six stressful situations. The potential negative impact on their life was investigated using 9 water parameters and 19 fish parameters including global serum protein. Proteinemia allows to follow important stress situation but it cannot be used to detect small variations within physiological limits +like stocking density. For stocking density proteinemia is at the limit of its discriminating power. Studies of protein panel modification in sera by classic chromatographic method permit in the pass to go further but shown their limits too. Recently proteomic approaches were used on fish by using either 2D electrophoresis and MALDI-TOF or SELDI-TOF to investigate panel and variation in difference type of fish tissues. The aim of the present study compare the effect of stocking density on sera protein panel of 2 groups of sea bass reared at 10 kg m-3 and at 100 kg m-3. Two approaches were used, the former for protein lower than 50 kDa by profiling on SELDI-TOF (Ciphergen’s protein chip system) on whole serum and the latter for the protein from 15 kDa to 250 kDa by LC MS-MS after separation by 2D electrophoresis. The SELDI-TOF profiling show two picks different (p<0.05), one increasing in 10 kg m-3 and the other in 100 kg m-3. The numbers of spots counted on gels (18-cm pH NL gd strips pH 3-10, and coomassie blue stained) are so different. The highest spot number for 10 kg m-3 is close from the lowest one in 100 kg m-3 (10 kg m-3:226-370 and 100 kg m-3:360-450). Synthetic gels show that 90/280 spots are present only in 100 kg m-3 and 17/204 spots only in 10 kg m-3 and the difference in spot expression is not taken in account. The gels from each group are globally homogeneous (Intra class:0.98>C.R..>0.93 in 10 kg m-3 and 0.95>C.R..>0.84 in 100 kg m-3) and the two groups are different. Heuristic clustering performed on all gels shared them into 3 classes. One including gels from 10 kg m-3 and 2 including 100 kg m-3 but one 10 kg m-3 gel. That confirms differences between gels of both group and lower homogeneity in 100 kg m-3. Twenty spot different were analysed by LR SM-SL and 3 spots were identified as Inter-alpha (Globulin) inhibitor H3, C3 complement & FBP32 and Warm temperature acclimatation related protein , proteins probably involved in inflammation. Those 2 methods permit to discriminate two different rearing conditions but within physiologic limit. Profile comparison can be a rapid and good discriminating method but only for small protein (50kd). The 2D electrophoresis and LC MS-MS is a long and heavy way to investigate difference in all the panel of protein until identification if necessary.