The identification of eosinophil colonies in soft-agar cultures by differential staining for peroxidase

There has been a need to easily quantitate the incidence of eosinophil colonies within soft agar cultures. This has been realized by layering of the agar with benzidine dihydrochloride that permits detection of peroxidase activity in cells. Eosinophil colonies can be specifically identified by the addition to the substrate of potassium cyanide, an inhibitor of enzyme activity in neutrophils and monocytes. The enumeration of eosinophil colonies can be accomplished by scanning fresh or embedded cultures with low power magnification.