Treatment of the isolated Sindbis virus envelope with proteolytic enzymes causes inactivation of the hemagglutinin (HA) but leaves the complement-fixing (CF) activity unaffected. Analysis by chromatography of the products of the trypsin digested envelope preparations (pre-labeled with different radioactive precursors) revealed: (1) the CF activity always separates as a single distinct peak; (2) the ‘glycopeptide’ part of the glycoprotein molecule separates as a homogeneous peak and has a molecular weight close to 14,000; (3) the CF activity is associated with the polypeptide, and not with the sugar part, of the glycoprotein molecule of the envelope. In spite of the low titer of HA in the trypsin-digest of the envelope, such preparations induce in rabbits the production of high-titer hemagglutination-inhibition antib