The probe diffusion of green fluorescent protein (GFP) in a strongly interacting host polyelectrolyte solution of sodium polystyrenesulfonate (NaPSS) was studied using fluorescence recovery after photobleaching (FRAP). Along with providing a wide range of background polyelectrolyte conditions for probe molecules by varying the NaPSS concentration from dilute to highly concentrated and using two different NaPSS molecular weights, 1 x 10(6) and 7 x 10(4) Da, we also varied the net negative charge of the GFP by varying the pH. The probe diffusion coefficient D was significantly greater (up to 15-fold) than expected from the Stokes-Einstein (S-E) relation in concentrated solutions of high molecular weight NaPSS, but only moderately greater (2-fold or less) for lower molecular weight polymer. The deviations from S-E behavior increased with increasing pH, i.e., increasing negative GFP charge. We conjecture that the strong deviation from S-E behavior is due not only to microviscosity and electrostatic effects but also (for high molecular weight NaPSS) to the viscoelasticity of the concentrated polymer solution. D was fitted to the stretched exponential equation D/D-0 = exp(-alpha c(nu)) at different pHs. In high molecular weight NaPSS solution the exponent v decreased with increasing pH, whereas for low molecular weight NaPSS solution it increased. Comparison with previous results shows that along with the concentration, size, flexibility, and charge of the polyelectrolyte background, probe charge plays a significant role in the diffusion of probe molecules.