1. Using the patch clamp method in the outside-out configuration, purinoceptor-dependent unitary currents were measured in tonsillar and transformed tonsillar human B lymphocytes. 2. Single channel currents were evoked by ATP4-, the free-acid form of ATP, and by 2',3' O-benzoyl-4-benzoyl-ATP (BzATP) in the micromolar concentration range, but not by 10 mM ADP3- or 0.5 mM Mg(2+)-bound ATP. 3. The channels could be activated and deactivated several times for as long as 30 min even in the absence of intracellular ATP, GTP, or glucose. 4. The channels were selective for small cations and had a conductance of 9 pS with Cs+ as the intracellular and Na+ as the extracellular monovalent cation. 5. The half-maximal activation of the channels was obtained by 114 microM ATP4- and by 16 microM BzATP. The increase in the open probability after raising the ATP4- concentration was mainly due to a decrease in the times the channels spend in the closed state. 6. It is concluded that human B lymphocytes possess cationic channels directly gated by extracellular ATP4-. Their agonist binding characteristics are typical for P2z purinoceptors, but their permeation behaviour is different from the large non-specific pores formed by ATP4- in fibroblasts, macrophages and mast cells.