The objective of therapeutic Extra Corporeal Blood Treatment (ECBT) is the removal or conversion of unwanted (mostly pathogenic) substances present in blood. At present ECBT is generally performed by using physical techniques such as dialysis, membrane filtration and centrifugation. These techniques are characterized by a low level of specificity leading to simultaneous removal of useful compounds. Administration of large quantities of substitution fluids (e.g., plasma-exchange, hemofiltration), is therefore often necessary. Higher levels of specificity can be obtained by using specific binders. Recent results obtained with synthetic polymers show the high potential of this technique. The use of biological substances, such as enzymes and antibodies, will enable an almost 100% level of specificity. It is obvious that the natural defence mechanism of the patient to be treated should not be triggered by the compounds used. This can be realized by either immobilizing the biological compounds on a solid phase, or by a physical separation between the compounds and the blood (e.g., by membranes). The former method seems more favorable since the efficiency will be optimal. The problems encountered, however, are numerous.