The proteins of cerebrospinal fluid and blood sera were studied in 20 unselected cases of schistosomiasis mansoni. It was found that the changes in the CSF protein fractions are secondary to the changes in the protein fractions of blood sera or result from damage of the blood — CSF barrier. This mechanism explained the changes in protein fractions of CSF in 6 patients. The changes in the blood — CSF barrier were atributed to the impairment of leptomeningeal vessels due to vasculitis that may occur in schistosomiasis. The gamma globulin content in the CSF was elevated in ten cases: in six it depended of high gamma globulin content in blood sera; in three it depended on changes in the blood — CSF barrier. In one patient the CSF gamma globulin content was suggestive of local inflammatory changes in the leptomeninges, possibly dependent on schistosomiasis mansoni.