A rapid centrifugal elutriation procedure was developed to prepare high amounts of pure human blood monocytes. After disruption by nitrogen cavitation, a cytosol and a membrane fraction were obtained by sucrose- Percoll density centrifugation. The plasma membrane fraction contained cytochrome b558 and was free of microsomal or mitochondrial cytochromes as determined by low-temperature absorbance spectroscopy. Cell-free NADPH-oxidase activity from monocytes and neutrophils was reconstituted with cytosol and membranes in the presence of sodium dodecyl sulphate. By comparison with neutrophils, the cell-free NADPH-oxidase from monocytes showed a two- to threefold lower specific activity. The NADPH- oxidase was reconstituted with neutrophil membranes and monocyte cytosol, and vice versa. The Km for NADPH was always lower when monocyte cytosol was used. These experiments indicate that the membrane- bound components of the NADPH-oxidase from neutrophils and monocyte are similar and that levels of NADPH-oxidase components in the cytosols differ. Interferon gamma-treatment of the monocytes had no effect on the specific activity of the cell-free NADPH-oxidase.