We study the effects on the leptonic asymmetries A_{FB}, A_{pol} and A_{LR} coming from a model of compositeness. We consider the effects coming from the self-energies and the vertex correction to $Z l^+ l^-$. Thus we use the Altarelli parametrization of the oblique corrections. We get the asymptotic limits of these corrections in terms of the parameters $(m^*, Lambda, f, f')$ and we get bounds for the quotient $m^*/Lambda$ with different values of $(f, f')$. We conclude that both asymmetries produce bounds for such quotient when $f'$ overweigth to $f$, and this fact is related with the breakdown of the custodial symmetry.