The demand for the reliable operation of rotating machinery in cryogenic applications has resulted in a continuing program directed toward determining the life and durability of ball bearings operating at cryogenic temperatures. On the basis of past work in this field, it was determined that the ball separator, when made from conventional materials, usually was the cause of failure. A program of screening separator materials with bearings subjected to actual operating conditions was undertaken. A number of different separator materials and arrangements were tested to determine their relative durability. The apparatus used provided a method of loading the bearing axially and included a means of measuring bearing torque, wear, and load during operation. All of the tests of this screening program were conducted using liquid nitrogen. The results are presented in the form of bearing life, progressive bearing torque, and progressive bearing wear for the various configurations and types of separator materials tested. Photographs showing the test apparatus and some test bearings are included in addition to a graphic presentation of the test results. This test program proved that a ball separator made from filled polytetrafluoroethylene material3is superior to the other separator materials tested in liquid nitrogen.*Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Boston, Massachusetts, October 17–19, 1960.