Blood loss in the Kolff twin-coil artificial kidney has been studied by radio-isotopic techniques using 51Cr and 59Fe and the results so obtained compared with the Hb and microhaematocrit methods. It has been shown that the true blood losses are considerably higher and more variable than those obtained by simple methods. The losses can be minimised by using Ultra-flo 100 coils (Baxter-Travenol) rather than Chron-a-coils (Baxter-Travenol), by altering the position of the coil during the wash-back procedure and by varying the composition and pressure of the wash-back fluid. The losses may be sufficient to account for the development of iron deficiency in patients on maintenance dialysis therapy.