Within biomonitoring it has been common practice to analyse the concentration of pollutants in organisms. However, focus is shifting towards an approach which combines chemical analyses with measurements of effects in organisms after exposure to contaminant. In this study, the objective was to develop and use non-invasive test methods within in vitro toxicity testing and biomonitoring in aquatic organisms living in contaminated areas. Resistance towards pollution has been shown in many organism. They have been shown to posses a mechanism named multixenobiotic resistance (MXR) consisting of a transmembrane pump which actively removes substrates out of cytosol, preventing accumulation. Pollution and environmental contaminant has been shown to induce or inhibit this defense. Hemocytes from two blue mussel species Mytilus edulis and M. galloprovincialis were used as biomarkers in this study. Bioassays to asses cell viability and MXR activity after in vitro exposure to the environmental contaminants brominated flame retardants, perfluorinated compounds and tributlytin or in vivo exposure to pollution were conducted. Within the toxicity testing levels of the contaminants hexabromocyclododecane (HBCD), tetrabromobisphenol A (TBBPA), perfluorononanoic acid (PFNA), perfluorooctane sulfonate (PFOS) and tributyltin (TBT) not far from concentration found in marine biota were used. The biomarkers success in biomonitoring were tested in a area with a known high degree of pollution in the Adriatic; Kaštela bay, Croatia. Hemocyte viability decreased after exposure to HBCD, TBBPA and TBT with regard to both metabolic activity and membrane integrity. Hemocytes sampled from male mussels seemed to be slightly more sensitive. An inhibition of the MXR activity were significantly increased after exposure to PFNA and PFOS. Female hemocytes seemed to posses a higher degree of MXR activity, supporting the possible enhanced tolerance towards environmental contaminants found within this study. Higher levels might prevent accumulation of xenobiotics, resulting in a decreased toxicity of accumulated contaminants. In the field study a reduction in cell viability and an induction of MXR activity were shown. The three parameters chosen showed different degree of sensitivity with MXR activity being most sensitive followed by metabolic activity and then membrane integrity in both laboratory and field studies. Hemocytes as biomarkers with cell viability and MXR activity as endpoints could be successfully used as they are easy to maintain and handle and react to low concentrations of contaminants in laboratory and field studies.