There is a wide range in the expression of major histocompatability complex (MHC) class II HLA-DR molecules on freshly isolated human peripheral blood monocytes from one individual. This study determined whether the variability in HLA-DR expression on monocytes correlated with their oxidative burst capability after treatment with either phorbol myristate acetate (PMA) or calcium ionophore A23187. Freshly obtained cells were incubated with 2',7'-dichlorofluorescin diacetate (DCFH-DA), a dye which is utilized for the determination of cell oxidative potential by flow cytometry. Cells were stimulated, and then stained with phycoerythrin (PE)-labeled monoclonal antibodies (mAb). Two-color flow cytometry was used to determine changes in DCFH-DA (green) fluorescence which corresponded to oxidative product formation by cells which stained only with the PE-labeled mAb (monocytes). While monocytes with a high or low surface density of HLA-DR differed substantially in HLA-DR expression, only small differences in oxidative capability between "high" and "low" subsets were observed. Thus, monocytes which are very different in HLA-DR expression possess very similar oxidative burst capabilities.