Imaging techniques, such as confocal microscopy and fluorescent activated cells scan are facilitating the study of responses at the single-cell level. Superoxide is reported to oxidise the non-fluorescent dihydrorhodamine 123 (DHR) to rhodamine 123. The generation of rhodamine 123 by human neutrophils, stimulated by the phorbol ester phorbol 12-myristate 13-acetate was inhibited slowly by diphenylene iodonium and rapidly by azide, but not by superoxide dismutase. In the absence of enzymes H2O2 (but not ) oxidised DHR slowly but the rate was greatly enhanced by peroxidases. The rhodamine 123 generated by phorbol-ester-stimulated neutrophils was observed to be located within the cell despite the fact that neutrophils failed to accumulate external rhodamine 123. This stimulated rise in cellular fluorescence was eliminated by excess extracellular catalase. It appears that H2O2, released on the outside, crosses the plasma membrane where oxidation of DHR is catalysed by cellular peroxidases. Since in a mixed population DHR failed to distinguish between -producing and non-producing HL60 cells it is not a suitable probe for single-cell observations. We conclude that DHR oxidation reports only the presence of H2O2 and intracellular peroxidases, and not the generation of by any one cell. Only peroxidase-containing cells fluoresce.