A method is described for the use of scanning electron microscopy on the surface of gold-labeled cells. It includes the use of 45- or 20-nm colloidal gold marker conjugated with Staphylococcal protein A. The marker is best recognized on the basis of its atomic number contrast by using the backscattered electron imaging mode of the scanning electron microscope. When the backscattered electron signal is mixed with the secondary electron signal, an optimum correlation between the distribution of the labeled sites and the cell surface structures is demonstrated. The method is illustrated by its application to the identification of human circulating granulocytes. Its good resolution, high contrast, and good labeling efficiency offers a promising approach to the specific localization of cell surface antigenic sites labeled with particles of colloidal gold.