The transport characteristics of a hollow-fiber blood ultrafilter were studied in vitro to provide an understanding of the factors which control solute removal rate and to permit design of a full-scale clinical device. The dependence of ultrafiltrate flux on transmembrane pressure difference, protein concentration, flow rate, and fiber geometry was correlated in terms of available theoretical analyses. Solute rejection was 1.0 for albumin and decreased to nearly zero for solutes of several thousand molecular weight. An analysis of overall hemodiafilter behavior showed that blood and ultrafiltrate flow rates of 200 ml. per minute can be attained with a device of reasonable size which would provide an inulin whole blood clearance of about 100 ml. per minute.