Cells of the green alga Selenastrum minutum have a high capacity for O 2 consumption mediated by extracellular peroxidase. The rate of peroxidase-mediated O 2 consumption is greatly stimulated by the addition of salicylhydroxamic acid (SHAM), and the O 2 consumption rate is further enhanced by Mn(II) in the presence of SHAM; Mn(II) has no effect on the rate of O 2 consumption by cells in the absence of SHAM. Both in the presence and absence of Mn(II), SHAM-stimulated peroxidase-mediated cellular O 2 consumption is inhibited by Cu(II). In the absence of Mn(II), inhibition is probably via chelation of Cu(II) by SHAM. The combination of Mn(II) and SHAM results if O 2 consumption even in the absence of cells in a peroxidase-free system. This latter O 2 consumption is 50% inhibited by catalase or superoxide dismutase, and completely inhibited by low concentrations of Cu(II). In this latter case, Cu(II) is not acting by chelation of the SHAM. Contrary to some reports in the literature, in S. minutum there is no evidence that the inhibition of peroxidase by Cu(II) is solely due to superoxide dismutase-like properties of Cu(II). Furthermore, it is suggested that the combination of Mn(II) and SHAM should not be used to activate peroxidase activity due to the potential for high rates of non-enzymatic O 2 consumption.