  An analysis of the Schwinger's action principle in Lagrangian quantum field
theory is presented. A solution of a problem contained in it is proposed via a
suitable definition of a derivative with respect to operator variables. This
results in a preservation of Euler-Lagrange equations and a change in the
operator structure of conserved quantities. Besides, it entails certain
relation between the field operators and their variations (which is identically
valid for some fields, e.g. for the free ones). The general theory is
illustrated on a number of particular examples
