A new approach for the control and interruption of enzymatic reactions via selective enzyme immobilization has been developed. The technique was exemplified by the use of three model enzymes with the corresponding macromolecular substrates: alpha-amylase/starch, trypsin/ insoluble collagen, and alkaline phosphatase/plasmid DNA. Prior to incubation with its substrate, each enzyme was provided with de novo thiol-groups by a two-step reaction involving N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP) and DTT. The chemical modification was achieved such that at least 80% of the native enzyme activity was preserved in all cases. In order to interrupt rapidly the reactions in which the enzymes were used, the modified enzyme was immobilized by reaction via its thiol groups on a thiolsulfinate-agarose derivative. The gel-bound enzyme could then be easily removed from unreacted substrate and product by filtration or centrifugation. Comparative studies showed that the immobilized enzymes had much lower activities in the reactions studied than the corresponding soluble ones. The potential for enzyme reuse was also demonstrated with the alpha-amylase derivatives, which were quantitatively released and eluted in fully active form from the agarose. We have shown that it is possible to achieve practically complete enzyme immobilization in short times and thus to control the progress of the reactions. Because of its simplicity and high efficiency, this approach may represent an interesting alternative for biotechnological processes involving macromolecular or solid substrates.