Soluble surface coat glycoproteins were purified by concanavalin A affinity chromatography from variant populations of Trypanosoma rhodesiense (Wellcome strain). Each variant yielded a glycoprotein consisting of a single polypeptide chain. The apparent molecular weights of the different glycoproteins ranged from 58 000 to 67 000. Charge heterogeneity analyses resolved from 1 to 3 closely spaced components with isoelectric points that were considerably different from variant to variant. Amino acid analyses revealed notable variations in amino acid compositions. Immunization of mice with purified glycoprotein protected them from homologous variant trypanosome infection. Hyperimmune sera raised to purified glycoproteins were obtained from rabbits and produced single precipitin lines in immunoelectrophoretic or immunodiffusion tests with homologous glycoproteins. No interaction could be detected in heterologous antiserum-glycoprotein combinations. Only variant homologous trypanosomes were agglutinated by antisera. Surface coat glycoproteins prepared from clone populations of variants were chemically and immunologically indistinguishable from the glycoproteins of original uncloned variants. The observed immunogenic specificity and chemical uniqueness of the glycoprotein preparations identify them as variant-specific surface coat antigens responsible for antigenic variability in T. rhodesiense.