THE PLANT disease known as tobacco mosaic, which was probably first recognized by Swieten (1857), was confused with a pock disease for about 35 years. Even Mayer (1886), who published a fairly accurate description of the symptoms of tobacco mosaic, did not recognize the fact that he was working with two different diseases. He did find, however, that the juice from diseased plants was infectious and that from healthy plants non-infectious. He also reported that twice filtering the infectious juice through ordinary filter paper rendered it noninfectious. In 1892 Iwanowski announced that the mosaic and the pock disease were in reality two separate diseases, and more surprising, that the juice from plants diseased with mosaic remained infectious after being filtered through a Chamberland filter. Although he could not demonstrate the presence of an organism in the infectious filtrate, he nevertheless regarded the disease to be bacterial in nature. Beijerinck (1898) confirmed Iwanowski's filtration experiments, but chose to regard the infectious agent, not as being bacterial in nature, but, as he wrote at the time, " as a contagious living fluid." He seemed to desire to convey the idea of an unorganized material, which was capable of growth in the presence of living cells. Iwanowski's filtration experiments of 45 years ago are generally regarded today as being the first demonstration of the group of infectious agents which we now call filterable viruses, or perhaps better, simply viruses. It is significant that he chose to regard the first of these agents as organismal in nature, whereas Beijerinck chose to regard it as a new type of infectious agent. This difference of opinion which arose 38 years ago has persisted down to the present time. Is tobacco-mosaic virus organismal in nature, or does it really represent some new type of infectious agent? Chemistry, then as now, was used as an aid in studying the nature of this virus. The year after Beijerinck announced his viewpoint, papers concerned with' the effect of various chemical reagents on the infectivity of tobacco-mosaic virus beoan to apnear. The earlier investigators, including Koning (1899), Heintzel (1900), Allard (1916, 1918), and Chapman (1917), appear to have been interested in obtaining an insight into the nature of viruses by studying the effect of chemical reagents on virus infectivity. Similar work was carried out later by Birkeland (1934), Duggar (1925, 1929), Fukushi (1930, 1933), Grant (1932), Henderson