The glycosyltransferases may be the evolutionary precursors of the immunoglobulins, although critical evidence for this hypothesis is not yet available. The transferases add sugars to non-glycosylated proteins, lipids, and organic molecules, as well as to oligosaccharides, glycoproteins, and glycolipids. The enzymes are specific, extremely polymorphic, occasionally inducible, and may be structurally related to one another. Circumstantial evidence links the transferases to both the MHC and T/t loci in the mouse. Finally, antibodies against purified transferases are difficult to produce and sometimes they react with immunoglobulins. The present hypothesis predicts that transferases should show some sequence homology with immunoglobulins, and that some MHC proteins will be glycosyltransferases.