In Friend murine erythroleukemia cells the presence of ion channels was investigated with the patch-clamp technique. During the first 48 hours after cell seeding, three types of ion channels, with the following order of membrane density, were found: i) a Ca2+-dependent K+ channel, fully activated at a cytosolic Ca2+ concentration of 10(-6) M and moderately activated at 10(-7)M; ii) a monovalent cation channel non voltage-activated, with an open-close kinetics dependent on the pressure gradient across the patch; iii) a chloride channel with a slow open-close kinetics. The latter two channels were labile and did not survive during intracellular perfusion. The membrane potential of the leukemia cells was not constant, but underwent large (tens of millivolts) fluctuations due to the opening of a few channels. The average resting membrane potential recorded in this study agrees with that measured in these cells by means of the accumulation ratio of the lipophilic cation Tetraphenylphosphonium.