An analysis of the three leptonic reactionse + e −→e + e −,μ + μ − andτ + τ − over a wide range of energy, 12<s√<46.78GeV is presented. The data were obtained with the JADE detector at thee + e − storage ring PETRA. They are compared to predictions of electroweak theories, in particular the standard model. For the total cross-sections of all three reactions and for the differential cross-section of Bhabha scattering no deviation from QED is found over the entire energy range. The differential cross-sections of μ and τ pairs at high energies show the angular asymmetry predicted by electroweak interference. The axial-vector and vector weak coupling constant, sin2 θ W andM Z are determined and compared to other measurements. Finally, limits on deviations from the standard model are given.