The differences in the protein composition of cyst fluids from tumors of the central nervous system have been explained by the conditions in which the exchanges between the blood and the cyst fluid occur as well by metabolic aspects related to the malignancy of the tumor cells. In order to explore the influence of these factors the protein fractions from cyst fluids of astrocytoma (13 cases) and from glioblastoma multiforme (10 cases) were studied and the results were compared to those obtained for the protein fractions of two other series of cases: the first was represented by the results obtained for protein fractions of cyst fluids from craneopharyngeoma (7 cases) and, the second, by those concerning to the fluids from inflammatory subdural collections (12 cases). The protein fractions were studied by paper electrophoresis and the results are presented in table 1. The participation of each globulin fraction was calculated and the unit for comparison was the percent value of the albumin fraction for each given case and the mean values and their standard deviations are presented in table 2. The data were discussed and it was possible to show that the differences found are more able to be explained on the basis of the conditions in which the protein exchanges between blood and cyst fluid occur possibly related to the structural characteristics of the wall of the cysts.