This paper traces the development of the specifications used to qualify inspection penetrants from their inception in 1956 by the U. S. Air Force as MIL-I-25135 through subsequent revisions from A to D, and then to the present day AMS 2644 specification. The methods of determining sensitivity were initially very simple and did not result in an assigned sensitivity level, which caused problems for users in deciding what materials to use. The change from the Air Force specification to the civilian specification took approximately 12 years, due to the difficulty in developing a satisfactory method of determining sensitivity levels and classifying penetrants in five levels. The discussion includes descriptions and photos of a number of the devices and instruments that were proposed, tested, and found to be unable to perform as desired. When a final test method was decided upon, statistical techniques were included in the specification to evaluate the test results

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