The purpose of this paper is to present information gained from a study of the lubricating-oil aspects of crankcase explosions. Experimental tests indicate the crankcase atmosphere of a Diesel engine is a potentially inflammable mixture of oil mist and air in which ignition may be initiated by an overheated part. The minimum ignition temperature of an oil mist/air mixture is reduced by decreased air flow, increased mixture temperature, and increased igniter size. No significant differences were found in the minimum ignition temperature of a wide variety of lubricating oils, even when diluted with up to 20 per cent Diesel fuel. As long as inflammable lubricants are employed, it appears that little can be done from a lubricating-oil or fuel standpoint to prevent crankcase explosions. It is indicated that the problem may best be attacked by further critical studies of operation and maintenance practices and continued refinements in engine design.