The binding of amtosera against synaptosomes from rat cerebral cortex to dissociated cells from both cerebral cortex and liver was assessed by immunofluorescent labelling. This showed that none of the liver cells bound antisynaptosome antibodies but that some of the cerebral cortex cells bound antibody on their surfaces. Immunofluorescent labelling showed that all the particles in the crude mitochondrial fraction from rat cortex bound antibodies present in the unadsorbed antisera. However, when the antisera were absorbed with purified mitochondria and myelin, only a proportion of the mitochondrial fraction particles then bound antibody. Isolated IgG from the adsorbed antisera was labelled with ferritin and incubated with the crude mitochondrial fraction. Examination in the electron microscope showed that the ferritin and hence the antisynaptosome antibody was bound to the post-synaptic thickenings of about 20% of synaptosomes having their junctions in the plane of section.