  We reexamine the derivation of relativistic transport equations for fermions
when conserving the most general spinor structure of the interaction and Green
function. Such an extension of the formalism is needed when dealing with {\it
e.g.} spin-polarized nuclear matter or non-parity conserving interactions. It
is shown that some earlier derivations can lead to an incomplete description of
the evolution of the system even in the case of parity-conserving,
spin-saturated systems. The concepts of kinetic equation and mass shell
condition have to be extended, in particular both of them acquire a non trivial
spinor structure which describe a rich polarization dynamics.
