Lymphocytes express voltage-gated (Kv) and Ca2+-activated (IKCa1) K+ channels. Recently, we found that WEHI-231, an immature B cell line, expresses voltage-independent K+ channels called large-conductance background K + channels (LKbg). Arachidonic acid (AA) has attracted attention because of its potential regulatory roles in the apoptosis of immature B cells. To elucidate the functional targets of AA, we investigated the effects of AA on membrane currents, voltages, and cytoplasmic Ca2+ concentration ([Ca 2+]c) of WEHI-231 and Bal-17 cells that represent immature and mature mouse B cells, respectively. In whole-cell patch clamp, both Kv and IKCa1 were inhibited by AA. On the other hand, AA activated LKbg current and non-selective cationic (NSC) current in WEHI-231 while only NSC current in Bal-17. Inside-out patch clamp study showed that AA directly activates LKbg. AA induced hyperpolarization of WEHI-231 and depolarization of Bal-17 cells, respectively. The selective functional expression of LKbg and their activation by AA were also confirmed in the immature B cells (B220+/AA4.1+) freshly isolated from mouse spleen. In fura-2 spectrofluorimetry, AA induced persistent increase in [Ca2+]c of WEHI-231 cells, which was attenuated by KCl-induced depolarization. In Bal-17 cell, however, AA induced only a transient increase of [Ca2+]c. In summary, the novel type of background K+ channels (LKbg) in immature B cells is strongly activated while the other K+ channels (Kv and IKCa1) commonly expressed in lymphocytes are inhibited by AA. The hyperpolarization and augmentation of Ca2+ influx by LKbg activation might play a role in the response of immature B cells to AA.