Protocols were developed for in vivo and in vitro characterization of the mass transfer performance of filters intended for use in membrane plasmapheresis. The protocols were applied to all presently available filters and also to secondary filters used in cascade filtration. Virtually no distinction was found in filtration rate or sieving coefficient of the ten plasma filters tested and all, except for one early model now considered obsolete, are clinically equivalent. In contrast, filters for cascade filtration varied widely in performance and still require further development for optimal use.