A novel process has been devised that uses the peritoneal membrane to remove plasma proteins from the body at a rate comparable to conventional extracorporeal plasma-pheresis. A vasodilator (4 mg histamine phosphate) is added to 1 liter of hypertonic solution (485 mOsmol/L), and infused intraperitoneally with a residence time of 4 hr. Plasma containing proteins is convected across the peritoneum through open pores into the cavity and removed. The next alternating infusion is with a hypotonic solution (214 mOsml/L) containing a vasoconstrictor (1.0 mg norepinephrine). This infusion restores the fluid removed from the subject in the previous exchange, and prevents the rapid fall-off in protein removal rates obtained with repeated infusions with vasodilators only. Peritoneal membrane plasmapheresis was successfully tested on canines, with protein removal rates of between 30% and 50% of total serum protein per day for 13 days over a 22 day period. Peritoneal membrane plasmapheresis represents a potentially inexpensive, continuous/nightly home treatment for protein mediated diseases treatable by extracorporeal plasmapheresis.