  The paper presents the new method of calculations of reaction amplitudes of
processes with spin 1/2 fermions. The method is based on the application of
isotropic tetrad in Minkowski space and basis spinors connected with it. We
obtained as test and illustration of the method the amplitudes of interaction
processes $e^-e^+\to f \bar{f} $, $e^- e^+\to W^-W^+ $ and one of the possible
diagrams of the process $e^+e^-\to e^+e^-e^+e^-e^+e^-$.
