  The general properties of the effective Hamiltonian for neutral meson system
improved by L.A. Khalfin in 1980 are studied. It is shown that contrary to the
standard result of the Lee--Oehme--Yang (LOY) theory, the diagonal matrix
elements of this effective Hamiltonian can not be equal in a CPT invariant
system. It is also shown that the scalar product of short, $|K_{S}>$, and long,
$|K_{L}>$, living superpositions of neutral kaons can not be real when CPT
symmetry is conserved in the system under considerations whereas within the LOY
theory such a scalar product is real.
