We study with the aid of direct-numerical simulations the influence of a solid-body rotation of spanwise axis upon turbulence in a channel. Rotation is shown to reduce the vortical activity in the cyclonic zone. In the anticyclonic region, hairpin vortices are reorganized by rotation, and become more and more coherent and inclined towards the wall as the rotation rate increases. One observes also in this region the formation of a constant mean-velocity gradient equal to the entrainment vorticity and, if the rotation rate is high enough, the disappearance of high and low-speed streaks at the wall.