Electrophysiological, immunocytochemical, and RT-PCR methods were used to identify a K+ conductance not yet described in MCF-7 human breast cancer cells. A voltage-dependent and TEA-sensitive K+ current was the most commonly observed in these cells. The noninactivating K+ current (I-K) was insensitive to iberiotoxin (100 nM) and charybdotoxin (100 nM) but reduced by alpha -dendrotoxin (alpha -DTX). Perfusion of (alpha -DTX reduced a fraction of I-K amplitude in a dose-dependent manner (IC50 = 0.6 +/- 0.3 nM). This DTX sensitive I-K exhibited a voltage threshold at -20 mV and was not inactivated. The time constant of activation was 5.3 a 2.2 ms measured at +60 mV. The averaged half-activation potential and slope factor values were 14 +/- 1.6 mV and 10 +/- 1.4, respectively. Immunocytochemical analysis demonstrated that plasma membrane was labeled by anti-Kv1.1 but not by anti-Kv1.2 nor anti-Kv1.3 antibodies. Furthermore, only Kv1.1 mRNA was detected in MCF-7 cells. Incubation in 1 and 10 nlM alpha -DTX reduced cell proliferation by 20 and 30%, respectively. These data provide the first evidence of Kv1.1 K+ channels expression in MCF-7 cells and indicate that these channels are implicated in cell proliferation.