A micro-fluorometric assay using 2',7'-dichlorofluorescein diacetate (DCFH-DA) to monitor oxidative burst (OB) in phagocytes has been developed. This assay is based on the oxidation of nonfluorescent DCFH-DA to highly fluorescent 2',7'-dichlorofluorescein (DCF) both intracellularly and extracellularly. A murine macrophage cell line, J774, and a human monocytic cell line, Mono Mac 6, were used as models. The cells were harvested from tissue culture flasks, washed, counted and adjusted to desired concentrations. They were then dispensed into a 96-well flat-bottom tissue culture plate. After adding DCFH-DA and an agent eliciting OB, the plates were incubated in 5% CO2 at 37 degrees C for various periods. The intensity of fluorescence was measured directly in the wells of the tissue culture plate with the cells in situ using a computerized microplate fluorometer at 485 nm excitation and 530 nm emission. This assay provided a rapid measurement of oxidative burst of phagocytes. The automated micro-fluorometric assay may be suitable for screening the immunomodulating activities of various biological and pharmacological substances.