A new Cysticercus cellulosae antigen is described and the results of complement-fixation reactions are presented. The antigen is prepared with vesicles of C. cellulosae from infected pig, washed several times with saline and dryed on filter paper. The dryed material is extracted with benzol for 24 hours at room temperature; the benzol is discarded and the extraction is repeated several times. The material is then dryed over calcium chloride and extracted with 20 volumes of absolute methyl alcohol during 25 days. The liquid collected after centrifugation (10,000 G, during 15 minutes at low temperature) is kept in ice box and used as antigen. The complement fixation test was standardized by the isofixation method for the quantitative technic. The results obtained with cerebrospinal fluid of patients suffering from cerebral cysticercosis and other conditions showed that the method used is a valuable test for the diagnosis of human cerebral cysticercosis.