A fracture mechanics technique has been used to determine the rate of fatigue crack propagation in rotating-beam SAE 52100 steel specimens in several lubricating environments. Experiments were conducted in a mineral and antiwear hydraulic oil under wet and dry conditions, a water-in-oil emulsion, distilled water, and air. Differences as high as 8:1 in crack propagation rates were observed among these environments; the differences were found to depend on the magnitude of the stress-intensity factor at the crack tip. A method is outlined for separating the crack initiation stage from the propagation stage in rotating-beam specimens. This method is expected to provide new insights regarding the chemical influence of lubricants on the mechanisms responsible for fatigue crack development in bearings.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Montreal, Canada, October 8–10, 1974