A study was conducted to determine: (1) the effectiveness of 0.22-mum and 0.45-mum pore size membrane filters in retaining microorganisms over a period of 72 hours of intravenous fluid administration; (2) the growth and multiplication characteristics of microorganisms retained on the filter; and (3) the possibility of pyrogens being released by microorganisms trapped on the filter. Lactated Ringer's injection was used as the i.v. solution. The 0.45-mum filter was found to be effective in retaining all microorganisms studied for at least 6 hours. However, Esherichia coli and Pseudomomas aeruginosa began passing through the 0.45-mum filter between 6 and 8 hours after seeding. All other microorganisms (Candida albicans, Citrobacter freundii, Klebsiella pneumoniae, Staphyloccus aureus) were retained for at least 72 hours. None of the microorganisms passed through the 0.22-mum filter during the 72-hour study period. With viable microorganisms on the filter for 72 hours, no detectable endotoxins or pyrogens were found in the filtrates as examined by the limulus and rabbit tests.