  We present a study of hopping conductivity for a system of sites which can be
occupied by more than one electron. At a moderate on-site Coulomb repulsion,
the coexistence of sites with occupation numbers 0, 1, and 2 results in an
exponential dependence of the Mott conductivity upon Zeeman splitting $\mu_BH$.
We show that the conductivity behaves as $\ln\sigma= (T/T_0)^{1/4}F(x)$, where
$F$ is a universal scaling function of $x=\mu_BH/T(T_0/T)^{1/4}$. We find
$F(x)$ analytically at weak fields, $x \ll 1$, using a perturbative approach.
Above some threshold $x_{\rm th}$, the function $F(x)$ attains a constant
value, which is also found analytically. The full shape of the scaling function
is determined numerically, from a simulation of the corresponding ``two color''
dimensionless percolation problem. In addition, we develop an approximate
method which enables us to solve this percolation problem analytically at any
magnetic field. This method gives a satisfactory extrapolation of the function
$F(x)$ between its two limiting forms.
