To examine the effects of supporting material and the properties of activated sludge, dynamic filtration of various activated sludges was performed at the filtration pressure of 0.3 kPa using non-woven fabrics with fiber density of 3.0 X 10(-2)-0.18 kg.m(-2) and stainless nets with pore size of 44-100 mum. No significant difference in initial flux was found between these non-woven fabrics and stainless nets, but the stainless nets kept higher flux for a longer time than the non-woven fabrics. Dynamic filtration of seven activated sludges with MLSS concentrations were 9.0-14 kg.m(-3) was performed using a stainless net with pore size of 77 mum. Average filtration resistance (alpha(av)) was estimated for each sludge, and their values ranged from 2.4 X 10(9) to 5.6 X 10(11) m.kg(-1). The alpha(av) value increased with decreases in average particle size of activated sludge in both the bulk (d) and the upper layer after sedimentation for 30 min (d(u)). It also increased with an increase in activated sludge concentration in the upper layer after sedimentation for 30 min (C-u). From these results, it was found that the values of d, d(u) and C-u were important factors determining the separation performance in dynamic filtration of activated sludge. When dynamic filtration of activated sludge at a sewage plant was conducted with a filtration area of 0.2 m(2) for more than 20 days, it was observed that flux and activated sludge concentration in permeate were strongly dependent on the C-u value.