Fusion of purified mouse sperm plasma membranes to planar lipid bilayers resulted in the insertion of three ion channel types. They could be discerned on the basis of their selectivity, conductance, gating and voltage-dependent properties. The presence of a previously reported large, Ca2+ -selective channel was confirmed. Here, it is reported that the Ca2+ -selective channel from mouse sperm plasma membrane displayed a PNa+/PK+ = 1.6 +/- 0.2 (n = 4) and was blocked by micromolar concentrations of ruthenium red. Fusion yielded also a cation-selective channel (PNa+/PK+ = 2.5 +/- 0.3, n = 3) with a main open conductance substate of 103 pS and a smaller open substate of 51 pS (600 mM K+ cis/100 mM Na+ trans). The channel inserted into bilayers in two stable fashions: a high-activity mode (open probability = 0.57 +/- 0.02, n = 3), and a low activity mode (open probability < 1%, n = 4). In high mode, the channel displayed bursting kinetics and burst length was voltage independent. In addition, a perfectly anion-selective channel, with a slope conductance of 83 pS (600 KCl cis/100 KCl trans), was identified. It displayed a high, nearly constant open probability (approximately 0.90) in the 0 to -80 mV range.