Wick's theorem for chronological products or its generalized version are used for the calculation of the scattering matrix in each order of perturbation theory [1-2]. The procedure is reduced to calculation of the vacuum expectation of chronological products of the eld operators in the interaction representation. As factors in these products a number of operators ai of the Fermi elds and the same number of their "conjugate" operators ai are considered. Here all continuous and discrete variables are included in the index. In the interaction representation the operators ai and ai correspond to free elds and satisfy the commutation relationships of the form [ai; aj ]+ = [ai; aj ]+ = 0; [ai; aj ]+ = Dij .