We experimentally investigate the influence of suspended neutrally-buoyant particles on the dispersion of a passive solute in pressure-driven axial flow in a constant aperture fracture (parallel plates configuration). A dye is employed as solute in order to measure its local concentration by means of a light transmission technique. In the experiments a dyed particle suspension displaces a transparent one at constant flow rate, for volume fractions φ ranging from 0 to 0.25 and for solute Péclet numbers (Pe(s)) between 35 and 476 (mean flow velocities U between 0.004 and 0.0544 cm/s). The local time variation of the solute concentration in the measurement zone was well-fitted by the solution of the advection-dispersion equation, and a longitudinal dispersion coefficient D for the solute was measured. For Pe(s)0) and without particles (φ=0) are equal within the measurement error. For Pe(s)>300, D decreases for φ>0 compared to φ=0. The magnitude of the reduction increases as φ increases, and also as Pe(s) increases. This decrease of D in the presence of suspended macroscopic particles is analyzed in the light of theoretical, numerical and experimental results from other authors that studied suspension flow in similar geometries.