  We present a preliminary study of the neutral kaon mixing bag parameter BK
using two flavors of dynamical Wilson fermions. We determine the matrix element
of the relevant DeltaS=2 operator by using both the conventional approach and
the so called ``non-subtraction method'', and find that the latter leads to
results with smaller uncertainties. After having implemented non-perturbative
renormalization, we study the dependence of BK on the see quark mass. At our
relatively heavy values of quark masses (0.60<M_P/M_V<0.75) such a dependence
is found to be negligible and the results, within the statistical accuracy, are
consistent with a quenched determination. As a preliminary result for the
renormalization group invariant parameter we quote BKRGI=1.02(25).
