The amt. of Cu in a substrate may be estimated either by interpretation of spore color or by dry mycelial wts. The former is most accurate with min. depths of media while mycelial wts. are preferable when larger vols. of media are used. Accuracy is not changed by varying surface area but depth of media influences spore color development. There is no apparent relationship between the bioassay for soil Cu and chemical and spectrographic analyses. Ionic Cu was found to be readily available to the mold, as was adsorbed Cu although development of spore color was retarded with the latter. Lab. technics and a procedure for routine assays of soil Cu are descr.