  The $S$-matrix is known to be independent of the gauge fixing parameter to
all orders in perturbation theory. In this paper by employing the pinch
technique we prove at one loop a stronger version of this independence. In
particular we show that one can use a gauge fixing parameter for the gauge
bosons inside quantum loops which is different from that used for the bosons
outside loops, and the $S$-matrix is independent from both. Possible
phenomenological applications of this result are briefly discussed.
