The feasibility of exchange of blood plasma in a three-channel, countercurrent convective-mass exchange apparatus (CCMEA) has been theoretically considered. Fundamental advantages of CCMEA over a blood plasma filter have been noted. Calculations based on a mathematical model show a possibility of attaining a 90% or higher degree of CCMEA plasma exchange in the absence of blood thickening and almost at any flow rate of the blood drawn from and returned to the patient, including a low flow rate, which is very important in procedures of this kind.