Well diffusion assays and two-dimensional gradient plates were of use in determining the effects of external pH on the toxicity of CuSO 4 , HgCl 2 and phenyl mercuric acetate towards a range of filamentous fungi. Such techniques also enabled assessment of the relative toxicities of the different compounds and differentiation of tolerant and sensitive species. For most of the organisms, a decrease in toxicity was evident with increasing pH.
INTRODUCTION• .