  We study quantized Yang-Mills theory with massive vector fields in the
framework of causal perturbation theory. The most general form of the
interaction which is invariant under operator gauge transformations is pointed
out. The generator of these transformations generally fails to be nilpotent.
This defect, however, is easily cured by including scalar fields in the gauge
transformations. Due to gauge invariance these scalar gauge fields couple to
the Yang-Mills fields with predicted strength. We also show that invariance
under ghost charge conjugation fixes the form of the interaction completely.
The coupling of the Yang-Mills fields and the scalar gauge fields to matter is
investigated. It is proven that gauge invariance implies unitarity of the
physical $S$-matrix. We always work in the Fock space of free quantum fields in
which all expressions are mathematically well defined.
