  Let X be a simply-connected closed oriented 4-manifold and A an embedded
surface of genus g and negative self-intersection -N. We show that for fixed
genus g there is an upper bound on N if the homology class of A is divisible or
characteristic. In particular, for genus zero, there is a lower bound on the
self-intersection of embedded spheres in these kinds of homology classes. This
question is related to a problem from the Kirby list.
