After planned immunization, a rabbit polyclonal antibody has been produced that recognized the Schiff base moiety of glycated proteins. Using an ELISA technique, antiserum specifically recognized reduced and glycated epitopes on a variety of proteins, and did not react with nonreduced and glycated proteins. The antiserum showed very low cross-reactivity with reduced but not glycated proteins. The assay was also used to determine the degree of ex vivo glycation of serum proteins from diabetic patients or hepatic cytosolic proteins from mice. Thus, this assay may be used to determine the amount of in vitro or ex vivo glycation of blood or tissue proteins.