  Theoretical consistency of the 3-3-1 model with right-handed neutrinos
demands that the number of family of fermions be exactly equal to three. In
this brief report we show that such theoretical requirement results in a clean
and severe bound on the Z-Z$^{\prime}$ mixing angle: $-3,979\times
10^{-3}<\phi<1,309\times 10^{-4} {with 90% CL}$.
