Comparative ultrastructural findings of meninges, ventricular ependyma and choroid plexus of mammals including man were summarized with the aim of a better understanding as to which cells can play a role as macrophages in the cerebrospinal fluid (CSF) cytology. Differences between phagocytic capacity of pial cells and arachnoid cells were emphasized. Comparative investigations showed that most of the cells in the CSF originate from the blood. Scanning electron microscopy of the subarachnoid space and cytological investigation of the CSF gave the result that haematogenous cells belong to the normal cell population of the subarachnoid space of normal individuals. These cells in the CSF react as their equivalents in the blood, that is, after confrontation with antigens the monocytes transform into macrophages, sometimes into multinucleated giant cells, the T and B lymphocytes transform into blast cells. On the basis of immunological and ultrastructural results, the classification of CSF cells in normal and pathological conditions has been proposed.