We have made stable complexes between wheat germ agglutinin and either 5 or 10 nm particles of colloidal gold. These complexes were phagocytosed by neuronal and glial cells in embryonic rat hippocampal cultures and the incorporated gold gave intense, low-background staining in the light microscope either directly, for the most heavily labelled cells, or after intensification by physical development of silver. Cells were labelled in a punctate fashion over perikarya and processes. In the electron microscope, particles of gold were observed in lysosomal vesicles, frequently in an aggregated form. Gold complex incorporated into cells in culture was retained by those cells over periods up to 20 days. Embryonic hippocampal cells were labelled in suspension culture by incorporation of wheat germ agglutinin-gold complexes and transplanted into the brains of syngeneic adult host rats. Grafted neurons and glia were observed in the electron microscope to retain high levels of gold label over periods up to 30 days. Receipt of synaptic connections by transplanted neurones was observed. Complexes of wheat germ agglutinin with 10 nm gold particles were injected unilaterally into field CA3 of the hippocampus of adult rats. Specific retrograde transport of gold was observed in the light and electron microscopes to pyramidal and hilar neurones of the contralateral hippocampus and to neurones of the medial septal nucleus. Colloidal gold-wheat germ agglutinin complexes appear to be useful cellular markers that can be visualized at both light and electron microscope levels.