This paper describes a fully passive telemetry technique based on microwave backscattering. In this technique, a subharmonically-pumped passive mixer is coupled to a bio-probe and one or two miniature antennas. When interrogated by an RF excitation, this device generates an amplitude modulated RF backscattering component centered at twice the frequency of excitation. An external sensitive receiver can be used to demodulate the backscattering component and recover the bio-potential. A simple prototype based on solid state diodes has been fabricated and tested for 2.4/4.8 GHz and has the dimensions of 11.5x4.6 mm2 and thickness of approximately 1 mm. Experiments with this very simple device show that low-frequency signals (fm<1 kHz) as low as 1 mV can results in double-sideband levels of greater than -126 dBm for an incident RF power of less than 1 mW/cm2. The proposed device is intended to be coated with an insulating bio-compatible coating and serve as a telemetry chip for chronic implantation inside the body.