Clear-water head and efficiency are generally lowered by the presence of solids, the concentration having a predominant influence on efficiency. Experimental results with two mine tailings products pumped at concentrations by weight of up to 60% indicated that the drop in head was limited to about 15% and the reduction in efficiency exceeded the reduction in head if the concentration by weight exceeded 40%. A similar pattern was found with another pump and a coarser mineral, however, the reductional effects were larger. With high concentrations of the finest tailings product (maximum particle size equals 0. 1 mm), the drop in efficiency was reasonably well correlated to the pump Reynolds number and the specific speed. Although the reductional effects are less pronounced in large pumps, the exact scaling criteria are not known at present.