When magmatic fractionation involves the settling and removal of crystals from the body of magma, the efficiency of the fractionation process may be defined as the degree of separation of the solid from the liquid phase. An expression is given that relates efficiency to the amount of mesostasis, or crystallized trapped liquid, in an igneous cumulate. The uranium contents of samples from a 349-m-long drill-core of part of the lower and hidden zones of the Skaergaard intrusion are used as a quantitative indicator of the amounts of mesostasis in the cumulates. There are marked changes in the amount of mesostasis over the length of the core and the average efficiency of fractionation was 85 %.