  Using the WKB approximation we perform a linear stability analysis for a
rotational flow of a viscous and electrically conducting fluid in an external
azimuthal magnetic field that has an arbitrary radial profile B_{phi}(R). In
the inductionless approximation, we find the growth rate of the
three-dimensional perturbation in a closed form and demonstrate in particular
that it can be positive when the velocity profile is Keplerian and the magnetic
field profile is slightly shallower than R^{-1}.
