Attempts to establish extremely low-frequency (ELF) threshold sensitivity limits in biological systems are presently based on estimates of thermal noise in the cell membrane. The Weaver-Astumian (Science 247:459-462, 1990) threshold (8 × 10-3 V/m) should in principle also apply to electric fields produced by Faraday induction. However, the 60-Hz magnetic field required to induce an electric field of 8 × 10-3 V/m is improbably large and at variance with the experimental facts, implying either that Faraday induction is not the mode of weak ELF magnetic field biointeractions or that such interactions have nothing to do with the cell membrane, which constitutes only 1% of the cell volume. We explore the possibility that magnetic field interactions are connected to the periodic changes in free calcium concentration associated with the cellular Ca2+ oscillator (CaO). Estimates of the free energy associated with the CaO reveal cyclic voltage changes of the order of 20 mV, suggesting that already existing electric fields within the cytoplasm may be capable of interacting with externally applied magnetic fields. We further hypothesize that CaO frequencies can be reinforced or driven into narrower passbands by weak external ELF signals acting on elements in the Ca2+ signaling pathway, e.g., via the calmodulin molecule.