The effect of low blood lead levels, BLL, on the apparent aggregability of human red blood cells, RBCs, and on the whole blood admittance was investigated by a method based upon the measurement of the dielectric properties of cellular suspensions.It was observed that in vitro lead exposure of blood alters the blood admittance measured 5 s after the end of shearing and the apparent RBC aggregability. These changes are observed at BLL ≥ 20 μg/dl. Examination of 30 blood samples displayed a marked inter individual variation in the lead effect. A comparison of the data obtained for suspensions in autologous plasma and in phosphate buffered saline testifies that the main source for this variation is due to inter individual changes in the plasma conductivity rather than to a different susceptibility of RBCs to lead.The experimental results obtained can be explained by the following mechanism: Lead adsorption on the membrane surface decreases the surface charge density and the potential of the RBC membrane, thereby reducing the electrostatic repulsion of RBCs. As a result, the size of the aggregates 5 s after the stoppage of shearing increases and the kinetics of their formation is accelerated. These lead induced alterations in the aggregation process are manifested in an apparent decrease in the aggregability and in the increased admittance.Qualitatively, the same alterations in the aggregability and in the admittance were also found in blood samples of six patients with moderately elevated BLL (27-43 μg/dl).The high sensitivity of the measured parameters to the lead concentration suggests that this method can be used for populations screening.