  We have shown that it is possible to formulate the consistent and
probability-preserving description of the $CP$-symmetry-violating evolution of
a system of decaying particles. This has been done within the framework of
quantum mechanics of open systems. This approach allows the description of both
the exponential decay and flavour oscillations. We have solved explicitly the
Kossakowski-Lindblad master equation for a system of particles with violated
$CP$ symmetry and found the evolution of _any_ observable bilinear in creation
and annihilation operators. The choice of a concrete observable can be done by
the proper choice of initial conditions for the system of differential
equations. We have calculated the evolution as well as mean values of the
observables most interesting from the physical point of view, and we have found
their lowest order difference with the $CP$-preserved values.
