Radioactively labeled macrophages were injected intracerebrally in order to acquire additional criteria for the identity of certain intracerebral cell types with cells of the monocyte-macrophage series. The intracerebral distribution of the labeled cells, their reactive ability following silver impregnation, and the formation of processes were considered as indications for ameboid motility. The localization, reactive ability, and structure of the cells were similar to that found in cells inside the brain which are considered to be monocyte derivatives, i.e., intraventricular cells such as epiplexus cells and supraependymal cells, progressive microglia, free subarachnoidal cells, and perivascular cells of intracerebral vessels. A survival time of 2 months was assumed for the cells since isolated, intracerebrally administered, peritoneal macrophages can still be demonstrated inside the subarachnoid space 2 months after the injection.