  This paper concerns instanton contributions to two-point correlation
functions of BMN operators in N=4 supersymmetric Yang-Mills that vanish in
planar perturbation theory. Two-point functions of operators with even numbers
of fermionic impurities (dual to RR string states) and with purely scalar
impurities (dual to NSNS string states) are considered. This includes mixed RR
- NSNS two-point functions. The gauge theory correlation functions are shown to
respect BMN scaling and their behaviour is found to be in good agreement with
the corresponding D-instanton contributions to two-point amplitudes in the
maximally supersymmetric IIB plane-wave string theory. The string theory
calculation also shows a simple dependence of the mass matrix elements on the
mode numbers of states with an arbitrary number of impurities, which is
difficult to extract from the gauge theory. For completeness, a discussion is
also given of the perturbative mixing of two-impurity states in the RR and NSNS
sectors at the first non-planar level.
