The claim that glycogen contains protein was first made exactly 100 years ago and has been the subject of contention ever since. It has now been established that rabbit-muscle glycogen contains a covalently bound protein of Mr 37,000, present in equimolar proportion to glycogen. The protein, named glycogenin, is joined to muscle glycogen via a novel linkage involving the hydroxyl group of tyrosine, a fact of possible significance in the light of insulin's message being transmitted by tyrosine phosphorylation. The protein is seen as the biogenetic precursor of glycogen. Its existence suggests an additional mode of regulation of glycogen metabolism since the amount, turnover and cellular location of glycogenin will influence the corresponding parameters for glycogen. A protein “primer” is suggested for the biogenesis of storage polysaccharides in general.