  We present a new class of constraints to the lepton-family number and
R-parity violating couplings from muonium conversion, $\mu^{-} + {_{22}^{48}}
Ti \to e^{-} + {_{22}^{48}} Ti$, a class of tau decays, $\tau \to l +$ (light
meson) with $l= \mu or e$, and $J/\psi$ and $\pi^0$ decays into a lepton pair.
We find that $\mu^{-} + {_{22}^{48}} Ti \to e^- + {_{22}^{48}} Ti$ provides one
of the strongest constraints along with $\Delta m_{K}, \Delta m_{B}$, $\mu \to
e \gamma$ and the neutrinoless double $\beta$ decay. Search for these
lepton-family number violating (LFNV) decays forbidden in the standard model is
clearly warranted in various low-energy experiments such as Tau-Charm Factories
and PSI, etc.
