The protein fractions of 10 fluids from cysts associated with tumours of the central nervous system were studied by paper electrophoresis; histologic diagnosis were those of astrocitoma (6 cases), ependimoma (one case), meningioma (one case) and craniopharyngioma (two cases). Results of electrophoretic analysis are summarised in table 2. The findings concerning astrocitomas were similar, specially when albumin and α-globulins were considered; in comparison to blood serum protein fractions, albumin content was found to be elevated and α-globulins content to be low. Differences among this group and the other cases of this series are illustrated in chart 4; the albumin content was lower in the case of meningioma and in those of craniopharyngioma; the low concentration of α-globulins was not found in these cases. Electrophoretic findings showed similar results in the case of ependimoma cyst fluid when compared to those of astrocitoma. Considerations are traced on the importance of systematic analysis of protein fractions of these cyst fluids and its significance as a diagnostic aid.