The adaptation of the reduction of nitro blue tetrazolium (NBT) by phagocytes to the measurement of the opsonising capacity of serum or milk whey is described. A dose-response curve of the absorbance of solubilised reduced NBT to the concentration of opsonins, along with a close correlation (r = 0.94) between the colorimetric NBT assay and ingestion of group B streptococci (as measured by reduction of [3H]thymidine incorporation by extracellular bacteria), was established. This NBT reduction test, employing 96-well flat-bottom microtitre plates, is simple, semiautomated, requires a low number of PMN, and is applicable to a large number of samples, which renders it useful as a screening test for the opsonising capacity of sera or other biologic fluids.