Resting cells of Escherichia coli, immobilized in a flow-through bioreactor, coupled the oxidation of formate or hydrogen to Tc(VII) reduction and removal from solution. Cells, pregrown anaerobically in a hollow-fiber membrane bioreactor, were challenged with 50 Β΅M Tc-(VII) in a carrier solution of phosphate-buffered saline. The radionuclide accumulated within the membrane component of the reactor, corresponding to the localization of the cells. Negligible Tc removal was noted in a reactor containing a mutant deficient in active Tc(VII) reductase, when supplied with formate as an electron donor. Formate or hydrogen was supplied as the electron donor for Tc(VII) reduction to cells immobilized in reactors operated in transverse (crossflow) and direct (deadend filtration) modes, respectively. Flow-rate activity relationships were used to compare the performance of the reactors. A flow rate of 2.4 mL h -1 supported the removal of 50% of the Tc from solution in a reactor operated in transverse mode with formate as an electron donor. In contrast, a flow rate of 0.7 mL h -1 , supported comparable Tc removal when hydrogen was introduced to a reactor operated in direct mode. The reduced reactor efficiency, when hydrogen was used as an electron donor, could be attributed, in part, to poor delivery of the gas to the cells. The biocatalyst was highly stable in the reactor; no loss in activity was noted over 200 h of continuous use.