  We analyze the vertex Zff in the framework of models that add a new bottom
quark in a nonsequential way and we evaluate the tree level contribution to
LEP/SLC data $R_{b}$, $R_{l}$ and the total Z width $\Gamma_{Z}$. We obtain
bounds for the mixing angles through the experimentally allowed contour regions
of the parameters $\Lambda_{L,R}$ introduced here. In order to get a more
restrictive region, we also consider the experimental results for $B\to \nu \nu
X$ as well.
