We consider a special class of dimension-six operators which are
assumed to be induced by some new physics with typical scales of
order \Lambda. This special class are the operators with two quarks
which can mediate transitions between quark flavours. We show that under
quite general assumptions the effect of these operators can be parametrised
in terms of six parameters, leading to a modification of the (at tree level
flavour diagonal) neutral currents and to an ffective CKM matrix''
for the charged currents, which is
not necessarily unitary any more. The effects of these operators
on charged and neutral currents are studied.
