  The rational homology balls $B_n$ appeared in Fintushel and Stern's rational
blow-down construction [FS2]. Later, Symington [Sy1], defined this operation in
the symplectic category. In [Kh2], the author defined the inverse procedure,
the symplectic rational blow-up. In this paper, we study the obstructions to
symplectically rationally blowing up a symplectic 4-manifold, i.e. the
obstructions to symplectically embedding the rational homology balls $B_n$ into
a symplectic 4-manifold. We prove a theorem and give additional examples which
suggest that in order to symplectically embed the rational homology balls
$B_n$, for high n, a symplectic 4-manifold must at least have a high enough
$c_1^2$ as well.
