A new, hybrid type of ultra-sensitive electrophysiological superoxide anion (O2*-) sensor is described, which is based on "membrane-engineered" mammalian cells immobilized in an alginate matrix. The membrane-engineering process involved the electroinsertion of superoxide dismutase (SOD) molecules in the membranes of Vero fibroblast cells, which acted as catalytic units able to convert O2*- to H2O2. Superoxide dismutation triggered changes to the cell membrane potential that were measured by appropriate microelectrodes, according to the principle of the bioelectric recognition assay (BERA). The sensor instantly responded to O2*- with a detection limit (S/N=3) of 100 pM. Combined with a 4-month storage capacity at room temperature, the novel biosensor principle offers new perspectives for monitoring ultra-low concentrations of free radical species and oxidative agents in biological systems.