The effect of arsenic on fungi is of special interest in that it has a direct bearing upon hygienic and commercial interests. Gosio (1892, 1897, 1901) found that certain of the fungi, _Mucor mucedo_ and _Aspergillus glaucum_, will grow on various arsenic compounds and exercise a reducing influence on them. These moulds attack all oxygen compounds of arsenic including copper arsenite, and develope arsenical gases. Sulphur compounds of arsenic are not influenced by these fungi. The same moulds would, if cultivated in soil containing arsenic, develope hydrogen arsenide. _Penicillium glaucum_ has such a strong and definite action on arsenic compounds that he states that there is no doubt of the possibility of poisoning by arsenical gas in a room hung with paper containing arsenic. The compounds are so extraordinarily potent that if a mouse is placed in a vessel in which the mould is strongly developed in the presence of arsenic, it dies in a few seconds. _Penicillium brevicaule_ uses the element in its development as a food substance. If material containing arsenic is placed in contact with dead fungi no reaction occurs. The life activity of the mould is evidently necessary for the reaction by which the arsenic-containing gases are liberated. Csapodi (1894) put forward the earlier results of Gosio and noted that the so-called arsenical fungicides do not only fail to kill the mould fungi but actually favour their development. This action explains why wallpaper containing arsenic is so disadvantageous in a room. Abba (1898) severely tested Gosio’s method of detecting arsenic by means of growths of _Penicillium brevicaule_, whereby arsenic gases are liberated, vindicating the method completely, and establishing the test as an exceptionally delicate one. Segale (1904) applied the same method to the detection of the presence of arsenic in animal tissues. Ono (1900) grew Penicillium cultures with solutions of potassium arsenate and found no important differences either of depression or stimulation. Orlowski (1902–3) stated that small doses of arsenic (1/1000–1/100% Sodium arsen--[11]) stimulate the growth of _Aspergillus niger_, larger doses up to ⅛% retard growth, while ⅙% kills. Spores of the fungus taken from soil containing arsenic are said to possess an immunity against arsenic, in that they germinate in the presence of an arsenic content which rapidly kills control fungi. This immunity is not specific for arsenic, but extends also to other poisons. The chemical composition and water content are not altered. [11] The exact compound is not specified in the abstracted paper, 1/1000–1/100% Natr. Ars. being given. _Conclusion._