Abstract: This invention provides a method for treating blood plasma wherein a hollow fiber membrane which comprises at least a skin layer on one surface of the membrane and also a porous layer inside the membrane is employed. The skin layer of the membrane has micropores with average pore size of 50 to 450 A, and the membrane shows a water permeability of 80 ml/m.sup.2.hr.mmHg or more, and permeabilites for human blood plasma albumin of 85% or more and for human blood plasma immunoglobulin G(IgG) of 80% or more, and a rate of inhibition against human blood plasma immunoglobulin M(IgM) of 40% or more. Use of the above mentioned hollow fiber membrane, which makes it possible to selectively remove immune complex, rheumatoid factors, etc., without decrease in levels of immunological functions, brings excellent effects on therapy of autoimmune diseases.