Sensitive effect determination, the understanding of molecular toxicity mechanisms and the discovery of novel biochemical biomarkers are some of the major challenges in ecotoxicology in dealing with chemicals in the environment. Among several ‘omics’ tools, proteomic approaches are used to study the whole proteome of organisms and may provide novel insights into the functional molecular state of a biological system and for discovery of new sensitive biomarkers indicating exposure or effects at low toxicant concentrations. In this study, a proteomic approach has been used in Mytilus galloprovincialis as a screening of changes in protein expression caused by a mixture of polychlorinated biphenyls (PCBs), in order to characterize the effects of these environmental contaminants on protein profile and to develop new molecular biomarkers through identification of more drastically altered proteins. To achieve this objective, 100 mussels were exposed to PCB 138, 153 and 180 for 3 weeks under controlled conditions at the concentration of 30μg/l. An equal number of mussels was kept under the same conditions, but not treated, as control. The edible parts were homogenized and lyophilized. Extracted proteins were quantified and separated by two-dimensional electrophoresis (2-DE). It has been made a comparative study of two-dimensional electrophoresis gels obtained from proteomic analysis and the changes in protein expression were assessed by image analysis. Image analysis included spot detection, quantification, normalization and matching. On average more than 1000 spots were resolved and altered expression was qualitatively detected. Stained protein spots of interest were excised from preparative gels and their tryptic digests were subjected to protein identification by mass spectrometry. It was used Matrix Science Mascot search engine, database NCBI and for a homology search the program BLAST. .. [edited by Author]