Colloidal interaction potentials induced by the overlap of mixed protein + polysaccharide interfacial layers, formed solely as a result of electrostatic attraction between these two biopolymers, have been calculated using the Self Consistent Field Theory. A significant difference between the nature and magnitude of these interactions, depending on the manner in which the charge is distributed along the length of the polysaccharide molecules, was predicted. For chains with an even distribution of charge, the repulsive interactions are in general weaker than those mediated by pure protein layers. For strongly charged polysaccharide chains, these become even attractive at a certain range of particle-particle separations. In part this is due to bridging by polysaccharides, occurring between opposite layers. However, in systems containing strongly charged polyelectrolyte, it is also the result of what in practice may be interpreted as a coacervate of protein + polysaccharide, with a tendency for aggregation, forming interfacial layers on the surface of the particles. In contrast, when the charge of the polysaccharide chains is unevenly distributed, the induced repulsive forces are much enhanced and become longer ranged compared to those for pure protein layers. Once the layers begin to overlap, the electro-steric interactions produced are found to completely overwhelm any van der Waals attraction, thus dictating the inter-particle interactions. We also present some preliminary calculations investigating the competitive adsorption of different polysaccharides onto the protein layer. The initial results, for polysaccharides of the same size and overall charge, suggest that the heterogeneously charged polyelectrolyte completely dominates the adsorption onto the surface, displacing all uniformly charged chains from the interface.