A unified classification of all families of elementary particles, that is, of baryons, Bose mesons and light fermions (leptons: μ, e, ν) is proposed. The classification is based on extension of the isobaric multiplet principle to leptons. One of the corollaries of the scheme is that the six leptons known at present are the only ones that may exist. In weak interactions leading to the production of leptons the change of the 3-projection of the isobaric spin is ΔT 3 = 0, ± 1 2 . When viewed from this angle the values of the ΔT 3 in various ways of hyperon β-decay indicate that many different modes of decay are possible, excepting a few cases that are specially mentioned. The selection rules for T 3 are not sufficient to explain the absence of weak interactions involving formation of various electrically neutral lepton pairs. The absence of such interactions can be explained by introducing for leptons a certain number N = Q− 1 2 m ( Q = electric charge, m = neutrino charge) and postulating the selection rule ΔN = ±1 for weak interactions involving lepton production.