A staining procedure has been developed for imaging actin-containing structures in thick plastic sections in the electron microscope. The stress fibres of a fibroblastic cell line were used as a model system, and were first characterized immunocytochemically. After fixation of cells in formaldehyde, mordanting in a solution of gadolinium chloride allows stress fibres to be stained for light microscopy with haematoxylin. A brief exposure to a solution of ammonium paramolybdate renders haematoxylin-stained structures sufficiently electron-dense to be imaged in 1 micron thick plastic sections in a JEOL 200CX electron microscope, operating at 200 kV, and possibly in conventional instruments operating at 100 kV, particularly if equipped with a lanthanum hexaboride source.