Adenosine 5'-triphosphate-(ATP)-induced whole-cell currents were studied in human B-lymphocytes, transformed by the Epstein-Barr virus, by means of the tight-seal voltage-clamp technique. During bath application of ATP, the membrane conductance was increased. The change of membrane conductance occurred within milliseconds. The dose response relationship for the ATP 4--elicited membrane current (/p) was fitted by the Hill function with a Hill coefficient of 1 and a KD value of 0.2 mmol/1. Adenosine, as well as the Mg2+-bound form of ATP, did not effect the membrane conductance. Iv did not desensitize within 1 min and could be evoked repeatedly up to 100 times in 1 cell in the presence of the G-protein blocker Guanosine 5'-o-(2-thiodiphosphate) (GDP IriS]). Therefore, it seems that ion channels in form of P2zpurinoceptors are involved in the observed effects. The permeability (P) sequence for cations carrying Ip was Pca:PK:Pcs:PN,:PTR~S = 35:2: 1.2:1:0.1. The reversal potential of [p was not changed by substitution of intracellular C1-for aspartate, indicating that anions are not involved in the purinoceptor-dependent conductance. A single-channel conductance of P2z-receptor-dependent ion channels of about 3 pS was determined by noise analysis of Ip.