  We report calculations of the parameter B_K appearing in the Delta S=2
neutral kaon mixing matrix element, whose uncertainty limits the power of
unitarity triangle constraints for testing the standard model or looking for
new physics. We use two flavours of dynamical clover-improved Wilson lattice
fermions and look for dependence on the dynamical quark mass at fixed lattice
spacing. We see some evidence for dynamical quark effects and in particular B_K
decreases as the sea quark masses are reduced towards the up/down quark mass.
