Abstract: There is provided an apparatus and method for selectively separating amino acids and defined proteins from the blood by enzymatic degradation, to obtain a therapeutic effect. The apparatus includes a closed system in which is mounted a membrane like structure presenting a wide surface area on which has been fixed the enzyme or enzymes necessary to effect the separation process together with heparin to render the surface non-thrombogenous. The method of selectively separating the amino acids and proteins resides in causing blood to pass from an artery in the patient over the active surface containing the enzyme and heparin and returning the blood to the patient via another blood vessel.