Copper is used extensively in the eradication of various types of disease-producing microorganisms, especially fungi, as well as other organisms which are a nuisance to water supplies, such as algae and certain animal forms. It is also known that traces of copper are essential for the growth of many of the lower forms of life. The extensive literature on the relation of copper to microbial development is largely limited to the above two phenomena. Comparatively little is known, however, of the effect of copper upon a mixed microbiological population consisting of many organisms with different metabolic processes. It is known, for example, that the growth of plants in certain soils, as peats, will respond markedly to the application of small amounts of copper (5). The development of fungi in copper-free media has been suggested as a means of determining the concentration of copper in a given soil; this method is based upon the response of A. niger to the presence of small amounts of available copper (3).