The references to the basophil type of granulocyte of the abnormal cerebrospinal fluid are strictly limited and deficient. Some authors describe them as tissue basophils (mast cells) stating that there are no basophil granulocytes in the spinal fluid. The purpose of this paper is to contribut to the study of this subject. The material of the study consisted of the clinical records of 300 neurologic patients whose spinal fluid cytologic examinations revealed the basophils. The results of these studies showed that the basophils of the cerebrospinal fluid and the blood basophils are morphologically identical. However, the lack of correlation between the number of basophils in the blood and in the spinal fluid suggests that the basophil of the spinal fluid comes from the leptomeninges and in a sense it is a tissue basophil. In the group of patients with inflammatory diseases of the nervous system (Group 1), mostly cases of lymphocytic meningitis, meningo-encepha- litis, and meningomyelitis, the basophils ranged between 0.1 and 18 per cent. In the cases of purulent and tuberculous meningitis the basophils were observed in a lesser number (0.1 to 5 per cent). In the group of patients with changes in the cerebrospinal fluid due to subarachnoid hemorrhage, brain cysticercosis and air injected into the subarachnoidal space (Group 2), the basophils ranged between 0.1 and 11 per cent. In several patients plasma cells and eosinophils were frequently observed in association with the basophil leucocytes. There was a good correlation between basofils and eosinophils in the group of patients with problable immune-allergic reactions (Group 2). Basophils disappear rapidly from the spinal fluid after the onset of the disease. Usually basophils are seen in cases with spinal fluid pleocytosis, but it is not uncommon to observe them in cases with normal cell count. We know nothing about the meaning of the spinal fluid basophil in relation to central nervous system diseases and this is a field open to clinical and experimental investigations. It would be tempting to suggest that basophils do appear in the spinal fluid as part of the cytologic changes which point out to an immune-allergic reaction in its acute phase.