The application of an immunoperoxidase method to labeling ofStreptococcus mutanssubsp.sobrinusfor scanning electron microscopy is described. Bacteria from four separate cultures were fixed in 2.5% buffered glutaraldehyde solution. The specimens were treated in accordance with an indirect immunolabeling procedure using a rabbit anti-S. mutanssubsp.sobrinusγ-globulin. Labeled bacteria were equipped with a layer of small, distinct globules of horseradish peroxidase reaction product. Bacteria exposed to control incubations were not equipped with this layer.