We introduce here a technique, "label-fracture," that allows the observation of the distribution of a cytochemical label on a cell surface. Cell surfaces labeled with an electron-dense marker (colloidal gold) are freeze-fractured and the fracture faces are replicated by plantinum/carbon evaporation. The exoplasmic halves of the membrane, apparently stabilized by the deposition of the Pt/C replica, are washed in distilled water. The new method reveals the surface distribution of the label coincident with the Pt/C replica of the exoplasmic fracture face. Initial applications indicate high resolution (less than or equal to 15 nm) and exceedingly low background. "Label-fracture" provides extensive views of the distribution of the label on membrane surfaces while preserving cell shape and relating to the freeze-fracture morphology of exoplasmic fracture faces. The regionalization of wheat germ agglutinin receptors on the plasma membranes of boar sperm cells is illustrated. The method and the interpretation of its results are straightforward. Label-fracture is appropriate for routine use as a surface labeling technique.

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