We distinguished four distinct groups of megakaryocytic cells on the basis of their voltage-gated membrane currents. One group of 32 cells (15%), exhibited an inward rectifying current and had a diameter of 12±3.5 μm (mean±S.D.). A large group of 85 cells (39%) exhibited only a `leakage-like' current and had a diameter of 15.8±3.7 μm. The other two groups of cells exhibited voltage-gated outward currents. One group consisted of 43 `I-type' cells (19%), with a diameter of 22.3±3.4 μm, for which the maximal outward current occurred for a voltage step from −60 to either 0 or +20 mV. For the last group of 60 `M-type' cells (27%), which had a diameter of 26.7±2.9 μm, the maximal outward current occurred for a voltage step from −60 to +80 mV, the largest voltage step used. The currents recorded in this study, from megakaryocytes having `leakage-like' currents and `I-type' currents, were indistinguishable from the voltage-gated currents of the megakaryocytes from myelogenous leukemia patients, in which voltage-gated currents were suppressed (Kapural, L., O'Rourke, F., Feinstein, M.B. and Fein, A. (1995) Blood 86, 1043), suggesting that the megakaryocytes from the myelogenous leukemia patients are a dedifferentiated or less mature form of megakaryocyte.