The distribution of the plasma proteins fetuin and transferrin in fetal sheep neocortex has been studied using the indirect immunoperoxidase method. Most of the cells (presumed to be neurons) in the early cortical plate (35 days gestation) were positive for fetuin. In contrast the fibre systems on either side of the cortical plate were positive for transferrin; some fibres were weakly positive for fetuin. It is proposed that these fibres may originate from brainstem and perhaps from some thalamic nuclei and that the plasma proteins are probably transferred from the neuronal cell bodies to their peripheral processes. It is further suggested that plasma proteins such as fetuin and transferrin may have a role in neuronal differentiation and synaptogenesis.