While no recognized industry standard currently exists for mycoplasma clearance testing of membrane filters, several methods have been used by membrane manufacturers and contract test laboratories. This validation exercise documents one approach to mycoplasma clearance testing and incorporates validation guidelines recognized in the biopharmaceutical industry today. The benefits of this method are (1) the consistent production of small monodisperse cells, 0.37 x 0.41 microm, at titers in excess of 1 x 10(9) CFU mL(-1) within 20-24 h of incubation; (2) the consistent detection of low levels of Acholeplasma laidlawii with an absence of false negative results; and (3) the ability to distinguish among membrane filters with different bubble points. The results of the validation exercise demonstrate that the equipment, materials, and test methods of the A. laidlawii filter challenge test are suitably understood and in a state of control. This method is appropriate to characterize the performance of membrane filters used in biopharmaceutical applications.