In the cell-mediated immune (CMI) system lymphocytes from sensitized animals incubated with antigen manufacture and release lymphokines which activate the hexose-monophosphate shunt in macrophages. The rate-limiting enzyme of this activation is NADPH oxidase, the activity of which can be quantitated by the amount of nitro-blue tetrazolium reduced to formazan, a blue precipitate. Data is presented which demonstrates that lymphokine-activated macrophages can be microscopically quantitated, both in the direct and indirect assays, by counting the number of macrophages containing formazan precipitate. The indirect component of this assay correlates directly to the skin test diameter. Further, it correlates better to the skin test than another assay for CMI, the macrophages aggregation factor assay. The simplicity and reproducibility of this assay provides another method whereby lymphokine activation of physiological events in macrophages can be determined.