A dialyzer comprising: first and second dialyzation chambers (2, 4), and an intermediate chamber interposed between said first and second dialyzation chambers (2, 4), wherein, each of said dialyzation chambers (2, 4) has opposed first and second ends; each of said dialyzation chambers contains a filter member that separates said chamber into a blood compartment (12, 14) and a dialysate compartment, each of said compartments extending between said first and second ends; said filter member in said first chamber is made of a filter material for filtering out plasma water from blood; said filter member in said second chamber is made of a filter material for passing dialysate from said dialysate compartment to said blood compartment (12, 14); said first chamber has, at said first end thereof, a blood inlet communicating with said blood compartment and a dialysate outlet communicating with said dialysate compartment; said second chamber has, at said first end thereof, a blood outlet communicating with said blood compartment and a dialysate inlet communicating with said dialysate compartment; said intermediate chamber extends between said second end of said first dialyzation chamber and said second end of said second dialyzation chamber and communicates only with said blood compartments (12, 14); said blood and dialysate inlets and outlets (40, 42, 44, 46)are located to produce dialysate flows in counter-current to blood flows in both of said chambers; and said first and second chambers (2, 4) are constructed to provide a dialysate flow path between said dialysate compartments at said second ends of said chambers charactised in that said dialysate flow path provides a constricted passage between said dialysate compartments, wherein said constricted passage is oriented to provide a flow path that forms an acute angle with the direction of blood flow in each of said blood Compartments (12, 14). first and second dialyzation chambers (2, 4), and an intermediate chamber interposed between said first and second dialyzation chambers (2, 4), wherein, each of said dialyzation chambers (2, 4) has opposed first and second ends; each of said dialyzation chambers contains a filter member that separates said chamber into a blood compartment (12, 14) and a dialysate compartment, each of said compartments extending between said first and second ends; said filter member in said first chamber is made of a filter material for filtering out plasma water from blood; said filter member in said second chamber is made of a filter material for passing dialysate from said dialysate compartment to said blood compartment (12, 14); said first chamber has, at said first end thereof, a blood inlet communicating with said blood compartment and a dialysate outlet communicating with said dialysate compartment; said second chamber has, at said first end thereof, a blood outlet communicating with said blood compartment and a dialysate inlet communicating with said dialysate compartment; said intermediate chamber extends between said second end of said first dialyzation chamber and said second end of said second dialyzation chamber and communicates only with said blood compartments (12, 14); said blood and dialysate inlets and outlets (40, 42, 44, 46)are located to produce dialysate flows in counter-current to blood flows in both of said chambers; and said first and second chambers (2, 4) are constructed to provide a dialysate flow path between said dialysate compartments at said second ends of said chambers charactised in that said dialysate flow path provides a constricted passage between said dialysate compartments, wherein said constricted passage is oriented to provide a flow path that forms an acute angle with the direction of blood flow in each of said blood Compartments (12, 14).