Summary. The intracellular potential of human peripheral blood lymphoeytes and monocytes was measured with a modified neurophysiologieal system and was found to vary between -20 mV and +20 mV. By depleting the total mononuelear fraetion of monocytes and Blymphoeytes, and by using various separation procedures to derive monoeyte, B-lymphocyte-, T-lymphocyte-, and null cell-enriched populations, it was possible to show that monoeytes and the majority of the B-lymphocytes had positive intraeellular potentials, whereas Tlymphoeytes and null cells had negative intraeellular potentials. Various factors, which include in vitro cultur