Nuclear reactor pressure vessels are clad with a layer of stainless steel approximately 6 millimeters thick. The characteristics of the steel of the pressure vessels are known to change with cumulative radiation exposure from the operating reactor. In situ nondestructive determination of the extent of the changes of the steel characteristics must be made through the stainless-steel cladding. Preliminary tests described here indicate that AC magnetic bridges can differentiate among steels of differing characteristics through stainless cladding.