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The distribution of the monoclonal antibody Cat-301 was examined in the cerebral cortex of macaque monkeys. Throughout the cerebral cortex, Cat-301 labelled the soma and proximal dendrites of a restricted population of neurones. The distribution was uniform within cytoarchitecturally defined areas (or subareas) but varied between them, with respect to the density of labelled neurones, the intensity of their immunoreactivity, their morphology, and their laminar distribution. Large numbers of intensely immunoreactive neurones were evident in motor related areas of the frontal lobe, somatosensory areas in the pareital lobe, and areas specialised for the analysis of visual motion in the parietal and occipital lobes. The heavily labelled areas are known to be interconnected, and the Cat-301 positive neurones within them were concentrated in the laminae from which their cortico-cortical connections arise. Given the critical role of somatosensory and visuospatial information in the execution of somatic and ocular movements, the heavily labelled areas may be regarded as sharing a broadly motor function. The timing of the expression of the antigen recognised by Cat-301 during development, its peri-synaptic location, and its biochemical characteristics suggest that it may play a role in the stabilisation of synaptic connections. Cat-301 may label networks of areas with a similar functional specialisation because the antigen plays such a role in relation to the specific interconnections that exist between them.