Abstract Measurement of the modulation of accumulation rate of model P-glycoprotein (Pgp) substrates has been a well established methodology for determination of the presence and activity of the multixenobiotic resistance (MXR) defence mechanism in aquatic invertebrates. Most studies have been focused on the gill tissue of various bivalves as a primary compartment for this type of measurements. In this study, we evaluated the potential of measuring the accumulation rate of a fluorescent model Pgp substrate rhodamine B (RB) in haemolymph, plasma and haemocytes of the freshwater painter's mussel ( Unio pictorum) as additional potentially useful compartments. The obtained results demonstrated several important advantages of the determination of Pgp mediated MXR transport activity in haemolymph over determinations in gill tissue. The overall MXR response correlated well with the level of Pgp activity simultaneously determined in gills. The method is more sensitive, the procedure is easier and less laborious, and repeated use of same individuals is possible. Finally — the approach is non-destructive, offering a potentially powerful biomarker and research tool for studies directed to the evaluation of ecotoxicological importance of MXR defence and the presence of MXR inhibitors in the environment.