Abstract: Sheep antiserum developed against the human glycohemoglobin, Hb A.sub.1c, distinguishes this fraction from the major component Hb A.sub.o. Partial cross relativity is observed with Hb A.sub.1a and Hb A.sub.1b, as well as with analogous glycohemoglobins from mouse and dog hemolysates. The reactivity of the hemoglobins with this specific antiserum is abolished by the reduction of the keto group of the sugar ligand. The immunological specificity displayed provides the basis of a quantitative assay for Hb A.sub.1c, which is suitable for studies of clinical and experimental diabetes.