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We have developed a SQUID-based technique to probe membrane-bound proteins, such as cation transporters. The method involves excitation of a biological sample with a sinusoidal electric field (0-5 V/cm, 10-300 Hz), detection, with a SQUID magnetometer, of the magnetic field produced by ac currents in the tissue or cell suspension, and calculation of the Fast Fourier Transform (FFT) of the measured response to obtain a spectrum. We find that the harmonic response of budding yeast cells (S. cerevisiae, 108 cells/ml) is sensitive to sodium metavanadate, an inhibitor of a proton pump known as H+-ATPase, glucose, a substrate of H+ -ATPase, and gramicidin, a membrane depolarizer. SQUID-based field and frequency dependent harmonic response measurements are also made of a live earthworm (Lumbricus terrestris). Use of the SQUID significantly reduces spurious harmonics and distortion of the excitation field by the measurement electrodes.