Paraffin oil droplets containing Oil Red O and coated with Escherichia coli lipopolysaccharide were ingested rapidly by human peripheral blood phagocytes only if they were pretreated with normal serum. This property formed the basis of a screening test in which lipopolysaccharide-coated particles were opsonized with patients’ serums and then were added to autologous leukocytes suspended in two portions, one of which contained nitroblue tetrazolium. After these incubations the cells were washed and extracted with dioxane. Oil Red O and nitroblue tetrazolium formazan in the dioxane extracts were spectrophotometrically assayed, thereby providing simultaneous determinations of the initial rates of ingestion and nitroblue tetrazolium reduction. The test differentiated opsonically-deficient serums and chronic granulomatous disease phagocytes from normals. Serums from individuals with bacterial infections had supernormal opsonic activity, and leukocytes from these patients had increased rates of nitroblue tetrazolium reduction in response to ingestion.