The copper(II) contents of the growth media, Sabouraud dextrose and Czapek–Dox broths, and of the spore inocula ofAspergillus niger(ATCC 1004),Aspergillus oryzae(ATCC 1011),Trichoderma viride(ATCC 8678), andMyrothecium verrucaria(ATCC 9095) were determined by atomic absorption spectrophotometry in a graphite furnace. The test systems composed of Sabouraud dextrose broth and spore inocula of the four fungi contained only a little over 3% of the copper(II) required to form a minimal inhibitory concentration of bis(8-quinolinolato)copper(II). The test system of Czapek–Dox broth andA.oryzaecontained slightly less than 65% of the copper(II) required to form a minimal inhibitory concentration of the bischelate of 8-quinolinol with copper(II). When the minimal inhibitory concentrations of 8-quinolinol and bis(8-quinolinolato)copper(II) were added simultaneously to the test system of Czapek–Dox broth andA.oryzae, 10% of the combined mixture of toxicants caused complete inhibition of growth indicating synergism between the toxicants. These results together with the observation that α-lipoic acid as well as small aliphatic thiol-containing compounds (cysteine, glutathione, dithioerythritol, and dithiothreitol) reversed the toxicity of 8-quinolinol but not the toxicity of bis(8-quinolinolato)copper(II) led to the conclusion that the mechanisms of fungitoxicity of both toxicants are different.