Two-Loop Effective Action for Theories with Fermions

On the basis of a new approach proposed in our previous work we develope a formalism for calculating of the effective action for some models containing fermion fields. This method allows us to calculate the fermionic part of the effective action (up to two-loop level) properly. The two-loop contribution to the effective potential for the Nambu-Jona-Lasinio model is calculated and is shown to vanish.