The cerebrospinal fluid (CSF) protein fractions were studied in 7 cases of Guillain-Barre syndrome by paper electrophoresis; in 5 of them the serum protein fractions were studied also. The investigation in each case was carried in the first month following the onset of the disease and prior to the beginning of therapeutics. There was a protein-cytologic dissociation in the CSF in all the cases. It is possible to classify the changes found in the CSF proteins as being similar to the "mixed type" pattern ("Mischelektrophoresediagramm", Bauer) with an associated increase in the participation of a-globulins. It was shown that the changes in the CSF protein fractions may be secondary to those that occurred in the serum and that they are dependent on disturbances in the ultrafiltration of blood proteins to the CSF. Such disturbances are possibly caused by the changes brought about by the disease in the blood-CSF barrier. The special behaviour of a1-globulin is shown by its relative content in the CSF. It was increased in 6 cases and its relation to the serum content was always superior to 1 (the maximum value observed was 4.8). The disturbances in the blood-CSF barrier are pointed out in the explanation of these findings, although the occurrence of a relative stagnation of this protein fraction in the CSF may play an important role also. Structural changes in this globulin and/or in the substances which are bound to it may modify the rate of its reabsorption, thus bringing about a relative stagnation.