The purpose of these experiments was to test two questions: (a) whether shortfibre asbestos is less damaging in the lungs of the rabbit than is long-fibre asbestos, and (b) whether metallic aluminium will suppress any toxic effect produced by either the shortor long-fibre asbestos. Early work with experimental asbestosis in animals has led Gardner, of Saranac Lake (Gardner, 1937, 1938, and 1940), to suggest a "mechanical" theory of asbestosis. In contrast to quartz and other forms of free silica, which appeared to become more toxic as their particle size decreased, the shorter the fibre of the asbestos dust used, the less tissue reaction was produced. In fact, asbestos dust which had been well ground in order to eliminate all long fibre produced almost no pathogenic effects in tissue at all. On the other hand, long-fibre asbestos, or the presence of long-fibre particles amongst dust of finer size, produced a diffuse fibrous response. The pathogenic effect with long-fibre asbestos, however, was obtained only in the lungs. In all other tissues asbestos of any length appeared to produce nothing other than a foreign-body reaction. These facts led Gardnei to conclude that it was only.in tissue where rhythmic movement takes place that asbestos can exert a pathogenic action, and then only if the particles are of sufficient size to produce mechanical irritation. * This work is a development of an investigation begun by the late Dr. T. H. Belt.