Injections of horseradish peroxidase (HRP) into the occipital cortex of the kitten and diffusing to the white matter label a widely distributed microglial population and in addition, cells with light and electron microscopic features of 'gitter cells'. The latter are concentrated in a complex and highly consistent system of interconnected clusters in the white matter of the lateral, postlateral, middle suprasylvian and posterior ectosylvian gyri, as well as on the roof of the lateral ventricle. The 'gitter cells' have the ultrastructural and (as described by others) chemical characteristics of macrophages, and may be involved in the elimination of transitory axons.