  The impurity concentration dependence of superconducting transition
temperature $T_c$ is studied within the framework of two band model of order
parameter anisotropy. Both intraband and interband electron scattering by
nonmagnetic and magnetic impurities are taken into account. It is demonstrated
that at various values of model parameters both types of impurity concentration
dependence of $T_c$ are possible: weak dependence typical of the model with
s-wave order parameter and strong suppression of $T_c$ by impurities with
critical impurity concentration at which $T_c=0$. It is found that in some
cases there is a possibility of a rise of critical temperature with increasing
impurity concentration. The obtained results are consistent with existing
experimental data for the high-temperature superconductors.
