A study was made of 220 samples of cerebrospinal fluid (CSF) from patients who suffered from several diseases of the central nervous system. In all samples immunological reactions for syphilis, cysticercosis, toxoplasmosis, Chagas' disease and schistosomiasis were studied comparatively. Immunofluorescent reactions for schistosomiasis were made by indirect antiglobulin technic with two types of antigen: the worm and the liver granuloma of hamster infected by Schistosoma mansoni. Emphasis is given on data concerning to 16 cases in which these reactions were reagent. The importance of routine search in the CSF for schistosomiasis antibodies is discussed. The concept of a 'CSF neuroschis-tosomiasis syndrome' is discussed as the main aspect of diagnosis "in vivo" of the disease. It is supported by the demonstration of specific antibodies in the CSF. Hypercytosis of lymphomononuclear type associated to the presence of eosinophil cells, protein concentration increase and gamma globulins increase are other characteristics found in the CSF in this syndrome.