  A natural SL(2,Z) invariant generalization of the Veneziano amplitude in type
IIB superstring theory is investigated. It includes certain perturbative and
non-perturbative (D-instanton) contributions, and it reduces to the correct
expressions in different limits. The singularities are poles in the $s$-$t$-$u$
channels, corresponding to the exchange of particles with a mass spectrum
coinciding with that of $(p,q)$ string states. We describe the general
structure of the associated perturbative corrections to the effective action.
