The electrical impedance parameters of human blood, that is, plasma resistance Rp, cell interior resistance Ri and cell membrane capacitance Cm, were determined by measuring impedance amplitudes at three different frequencies, 0.1, 0.8 and 1.2 MHz. Several new findings have been obtained. The fibrinogen in normal blood raised Rp and Cm by about 4% and 20%, respectively, and serum proteins contributed to the capacitance by about 14%. The results imply that the electrical impedance of blood may reflect certain diseases that involve abnormal compositions of certain plasma proteins. Measurement on 62 samples with various erythrocyte sedimentation rates (ESR) demonstrated that both Rp and Cm were proportional to ESR, implying that the impedance measurement might be an alternative method for quick estimation of ESR. During in vitro storage of blood at 4 degrees C, both Rp and Cm decreased with time, about -20% for Cm after four weeks of storage. The results imply that the impedance change might be a useful index for evaluating the quality of stored blood.