We study the transition from laminar to turbulent flow in a channel seeded with finite-size neutrally buoyant particles. A fixed ratio of 10 between the channel height and the particle diameter is considered. The flow is examined in the range of Reynolds numbers 500 ? Re ? 5000 and the particle volume fractions 0.001 ? ? ? 0.3. We report a non-monotonic behavior of the threshold value of the Reynolds number above which the flow becomes turbulent, in agreement with previous experimental studies. The mean square velocity fluctuations and Reynolds shear stress of the fluid phase are reduced by increasing the particle volume fraction at a fixed Re=1500, while the mean square velocities of the solid phase are enhanced monotonically suggesting a transition from fluid to particle dominated dynamics at high volume fraction.