The aim of this study is to perform ultrafiltration experiments in vitro with several commercial hemofilters for continuous renal replacement therapy (CRRT) in aqueous albumin solution and in various modifications of bovine blood in order to discuss separation characteristics of albumin. In aqueous solution system, 2000 mL of a single component albumin solution (2.552 g/L) was used. Flow rates of the albumin solution and ultrafiltration were 100 and 10 mL/min, respectively, and the experiment was performed for 720 min at 310 K. Adsorption of albumin was evaluated from the time courses of the concentration profile and sieving coefficient (s.c.). We investigate two polymethylmethacrylate (PMMA), two polysulfone (PS) and one polyacrylonitrile (PAN) hemofilters with four kinds of bovine blood test solutions as well as the aqueous solution. PMMA that has strong adsorption characteristics showed relatively low s.c. for albumin (0.01-0.03) in aqueous albumin solution. PMMA membrane with enlarged pore size showed two times higher s.c. for albumin than that with normal pore size. Albumin concentration in the reservoir in aqueous experiments decreased approximately by 20% and 50% in 630 min using the PMMA membranes with normal and enlarged pores, respectively. On the other hand, albumin concentration decreased by only 5.0% and 4.4 to 5.5% using PAN and PS hemofilters, respectively. The discrepancy may be explained by the physical structures of the membrane, i.e. albumin molecules may have been occlusively trapped by the entirely dense structure of PMMA. In the PS membrane, s.c. for albumin in the aqueous system was not significantly different from those found in various bovine serum systems which implied that a very small amount of albumin was adsorbed by the PS membrane due to reduced hydrophobicity caused by the use of a large amount of hydrophilic agent.