A membrane filtration technique with commercially available membrane filters (Millipore Corp.) was effective for the removal of Reiter treponemes from liquids such as fluorescent-antibody conjugates, to which the organisms are added for adsorption. Reiter treponemes from an 8-day culture were not microscopically detectable in filtrates through membranes with a pore diameter of 0.45 mum, but treponemes were demonstrated in the filtrate by cultural methods. No organisms of the 8-day culture passed through a membrane filter having a pore size of 0.22 mum, as determined by microscopy and culture. Culture data indicated that a filter with a pore size of 0.1 mum was necessary to prevent passage of treponemes from 4-day cultures. It is recommended that a membrane filter with a pore size of 0.22 mum or smaller be used for the removal of Reiter treponemes from suspensions and that the age of the culture be considered in choosing filter pore size.