  A known generalization of the Stillinger-Lovett sum rule for a guest charge
immersed in a two-dimensional one-component plasma (the second moment of the
screening cloud around this guest charge) is more simply retrieved, just by
using the BGY hierarchy for a mixture of several species; the zeroth moment of
the excess density around a guest charge immersed in a two-component plasma is
also obtained. The moments of the electric potential are related to the excess
chemical potential of a guest charge; explicit results are obtained in several
special cases.
