The permeation of DNA through regenerated cellulose hollow fibers that have a mean pore diameter of 15 nm was investigated in the absence of protein at a transmembrane pressure of 200 mmHg at 25 degrees C. The membranes can reject both double stranded DNA and single stranded DNA to some extent when the DNA concentration of the feed solution is 10-100 ppm. It was found that single stranded DNA can permeate through the membranes preferentially over double stranded DNA. This is explained by the higher flexibility of the single stranded DNA over the double stranded DNA and is not mainly due to a different distribution of molecular weights between the single stranded DNA and the double stranded DNA. The circular dichroism spectra of feed and permeate solutions suggest that the DNA was not denatured by permeation through the membranes.