Salicylhydroxamic acid (SHAM), an inhibitor of the alternative respiratory pathway, reacts with 2', 7'-dichlorofluorescin diacetate (DCFH-DA) and fluorescein diacetate to form 2', 7'-dichlorofluorescin and fluorescein, respectively. These reactions show that SHAM possesses esterase-like activity. As SHAM may act on endogenous acetoxymethyl ester-type compounds, assessment of respiration patterns of cells requires cautious interpretation, especially during prolonged incubation.