In this paper Membrane Bioreactor (MBR) activated sludge is obtained from full scale MBRs and submitted to cross-flow filtration using always the same membrane and operational conditions. Samples are diluted in order to create suspensions with different Mixed Liquor Suspended Solids (MLSS) concentrations. MLSS values varied between 3.6 and 18.3 g/L Contrary to literature results filterability did not present a continuous increase with decreasing MLSS concentrations. Activated sludge with MLSS concentration exceeding 10 g/L originated dilution samples with worse filterability. The dilution caused an increase of particles in the range 10 to 20 mu m and a decrease in the range 30 to 100 mu m. The activated sludge samples had non-Newtonian viscosity and Soluble Microbial Products (SMP) concentrations varied between 60 and 11 mg/L Activated sludge samples with MLSS concentrations exceeding 10 g/L are entrapping particles smaller than 20 mu m in the activated sludge bulk that therefore are not contributing to the increase of resistance on the membrane. Activated sludge samples with lower MLSS concentrations do not show the capability to entrap particles. These different behaviours suggest that are optimal MLSS concentration ranges for MBR operation. (C) 2009 Elsevier B.V. All rights reserved.