The cytotoxic effects of a series of non-asbestiform fibrous and other mineral dusts towards mammalian macrophage and non-macrophage cell lines in tissue culture have been examined. It is considered that such tests may be useful indicators of the Pathogenicity of dusts in vivo , although many additional factors may also influence the latter. The indications are that dusts that are non-cytotoxic to either cell type are non-pathogenic in vivo , those toxic to macrophages are fibrogenic in vivo , and those toxic to both the macrophage and the non-macrophage cells may be both fibrogenic and carcinogenic.