A membrane of microporous polyethylene hollow fibers (EHF membrane) has often been used to prepare sterilized water in medical institutions. In the present investigation, we have studied the capacity of EHF membrane to adsorb mutagens from drinking water and examined some properties of the mutagens. Through the module of EHF membrane, drinking water of three cities (A, B and C) was passed and the adsorbate extracted with methanol. The methanol extract was dissolved in dimethylsulfoxide and tested for the mutagenicity to Salmonella typhimurium TA 100, TA 98 and TA 1537 with and without S9 mix. All the extracts showed mutagenic activities to these three tester strains, and it was demonstrated that the EHF membrane has a capacity to adsorb mutagens from drinking water. The mutagenic activities of the extracts decreased by the addition of S9 mix to these test systems. When the adsorption ability of EHF membrane was compared to that of the activated charcoal, the ability of the former was about 4.6 times higher than that of activated charcoal. Mutagens adsorbed of EHF membrane were soluble in most organic solvents, especially in ethyl ether. When the methanol extract was separated into acidic, basic and neutral fractions with ethyl ether, the neutral fraction showed the highest mutagenic activity. The mutagens adsorbed to EHF membrane were relatively stable to heat.