Many normal cerebrospinal fluids contain an alkaline (pH 9.8) and an acid (pH 4.9) phosphatase. Both the alkaline and the acid phosphatase were significantly increased in the spinal fluids from patients with meningitis or poliomyelitis, but not in the fluids from patients with syphilis. The alkaline phosphatase activity was correlated with both the concentration of protein in the spinal fluid and with the white cell count, whereas the acid phosphatase was correlated with neither. When correction was made for the significant correlation between cell count and protein concentration, the partial correlation between alkaline phosphatase activity and both protein concentration and cell count remained significant statistically. In pathological conditions it appears likely that the alkaline phosphatase is derived partly from the polymorphonuclear leucocytes in the fluid and partly from the blood plasma. The acid phosphatase is probably derived from the lymphocytes of the fluid and possibly also from the blood plasma. It is unlikely that either of these enzymes comes from the substance of the brain or spinal cord. Acid phosphatase would be of more value than alkaline phosphatase as a diagnostic aid, since normal fluids contain much less of this enzyme.