The time course change in particle penetration through capillary pore membrane filters was investigated for relatively short filtration periods. In the laboratory experiments, penetration efficiencies of the 0.3 and 1.0 mum standard particles through the 2.0 mum capillary filter were continuously monitored. While penetration efficiencies of both 0.3 and 1.0 pin particles decreased, 1.0 pm particles became selectively captured by the filter. Furthermore, even small amount of collected particles (coverage fractions less than 1% of the filter surface) was found to alter particle penetration characteristics possibly because of the intense accumulations around the rims of capillary pores due to significance of particle interception. No distinct effect by addition of clogging particles of 2.0 mum diameter was observed, indicating the change in particle penetration characteristics was initiated even when particle clogs became important. The present study showed the size distribution of collected particles by capillary pore membrane filters could be biased with time even for short sampling duration. (C) 2003 Elsevier Ltd. All rights reserved.