Interactions of blood cells (RBCs) with a microelectrode of 50 microns diameter have been examined under flow conditions using impedance measurements at high frequencies. At such frequencies, the electrolyte resistance (Re) is assimilated to the real part of impedance, and interactions are associated with transient fluctuations of Re. Sedimentation experiments suggest that one erythrocyte contributes to a 1.1% Re increase. Effects of wall shear rate (from 25 to 140 s-1) and RBC concentration (from 8.4 x 10(5) to 2.7 x 10(6) cells/ml) have been investigated; the number of interactions rapidly decreases with wall shear rate. Event frequency is proportional to RBC concentration ranging from 3.1 x 10(6) cells/ml to 1.3 x 10(7) cells/ml. At high concentrations of RBCs, some transient events overlap. Videotaped images help to determine how many RBCs interact with the microelectrode at the same time on separate surface areas. Under flow conditions, the contribution of one RBC on the Re increase is similar to the mathematical value obtained by sedimentation and decreases slightly with wall shear rate.