The introduction of new fracture propping agents that are. brittle but much stronger thun sand creazed the problem o! what loading strength is required for a propping agent to be efiective in a given jormation. It is shown that the load at which the propping agent crushes shouid exceed the ioad at which total embedment in the jracture fuces is possibie. Simpie labor~rory tests to determine loading strength of the propping agent and embedment in the fracture faces, and use of these data in seiecting a prop-ping agent for a given formation, are discussed.