A new membrane af®nity biosorbent carrying thionein has been developed for selective removal of cadmium ions from human serum. Microporous poly(2-hydroxyethyl methacrylate) (pHEMA) membranes were prepared by photopolymerization of HEMA. The pseudo dye ligand Cibacron Blue F3GA (CB) was covalently immobilized on the pHEMA membranes. Then, the cysteine-rich metallopeptide thionein was conjugated onto the CB-immobilized membrane. The maximum amounts of CB immobilized and thionein conjugated on the membranes were 1.07 mmol cm À2 and 0.92 mmol cm À2 , respectively. The hydrophilic pHEMA membrane had a swelling ratio of 58% (w/w) with a contact angle of 45.8 °. CB-immobilized and CB-immobilized±thioneinconjugated membranes were used in the Cd(II) removal studies. Cd(II) ion adsorption appeared to reach equilibrium within 30 min and to follow a typical Langmuir adsorption isotherm. The maximum capacity (q m ) of the CB-immobilized membranes was 0.203 (mmol Cd(II)) cm À2 membrane and increased to 1.48 (mmol Cd(II)) cm À2 upon CB±thionein-complex conjugation. The pHEMA membranes retained their cadmium adsorption capacity even after 10 cycles of repeated use.