Following his mental collapse at the end of the nineteenth century the paintings of Ralph A. Blakelock suddenly began to bring exceptionally high prices. Not surprisingly, therefore, many forgeries attributed to him were produced in the early twentieth century. In fact, some experts consider him to have been the most widely forged American painter. At Brookhaven National Laboratory a group of paintings attributed to him have been studied by thermal neutron activation autoradiography combined with solid state high resolution gamma-ray spectroscopy. Most of the paintings studied were of unquestioned authenticity coming from the collection of the Metropolitan Museum of Art, the National Collection of Fine Art of the Smithsonian Institution, the Sheldon Memorial Art Gallery of the University of Nebraska, and two private collectors with well established histories of acquisition. These included paintings produced by Blakelock both before and after his breakdown. In general a number of characteristic consistencies in the types of pigments and media used and how they were employed in these paintings were revealed by the activation study. Of course, major differences exist between his early and later work. However, a lack of these characteristics and hidden overpainted details which are not revealed in conventional X-ray radiographs have provided evidence that two of the paintings attributed to Ralph Blakelock are the work of other artists. Some paintings of Blakelock's contemporaries George Inness and Albert Ryder have also been investigated.