  We consider type IIB string interaction on the maximally supersymmetric
pp-wave background and discuss how the bosonic symmetries of the background are
realized. This analysis shows that there are some interesting differences with
respect to the flat-space case and suggests modifications to the existing form
of the string vertex. We focus on the zero-mode part which is responsible for
some puzzling string predictions about the N=4 SYM side. We show that these
puzzles disappear when a symmetry preserving string interaction is used.
