  We calculate the radial diffusion coefficient for a passive contaminant in an
accretion disc which is turbulent due to the action of the magnetorotational
instability. Numerical MHD simulations are used to follow the evolution of a
local patch of the disc using the shearing box formalism. A separate continuity
equation for the mass fraction of contaminant is integrated along with the MHD
system, and radial profiles of this fraction are obtained as a function of
time. Solutions of a linear diffusion equation are fitted to the numerical
measured profiles of the contaminant, treating the diffusion coefficient D as
the fitting parameter. At early times, the value of D is found to vary, however
once the contaminant is spread over scales comparable to the box size, it
saturates at a steady value. The ratio of D to the transport coefficient of
angular momentum due to shear stress is small. If D can be used as a proxy for
the turbulent magnetic diffusivity, the effective magnetic Prandtl number
P_eff=\nu/D (where \nu is the coefficient of "efective viscosity" associated
with shear stress) would be large.
