The technique of cross-linking with a series of bifunctional diimidates (maximal effective length ranging from 3.7 to 14.5 A), followed by dodecyl sulfate gel electrophoresis, was applied to compare the subunit contact areas of phosphorylases b, ab (the phospho-dephospho hybrid), and a and to study the structure and ligand-induced structural changes in phosphorylases ab and a. Similarly to phosphorylase b, the nearest cross-linkable lysyl-NH2 groups are about 3.7 A apart across the intradimer subunit interface (contact m) and about 8 A apart across the interdimer interface (contact d) in both phosphorylases ab and a. The activation of phosphorylase b induced by phosphorylation and that elicited by AMP binding are distinguishable at both contacts m and d. Phosphorylases ab and a tend to form tetramers whose structures are not identical, but AMP renders phosphorylase ab similar to phosphorylase a. Glucose, caffeine, and glycogen are able to dissociate both a and ab tetramers to dimers, whereas glucose 6-phosphate can only dissociate phosphorylase ab. The structure around the nucleotide site of phosphorylase a is rigid so that ligands binding here, such as AMP, ATP, ADP, inosine monophosphate, and glucose 6-phosphate, fail to influence the cross-link pattern. In control, in phosphorylase ab contact m is markedly affected by AMP, ATP, and glucose 1-phosphate; hence, in this respect phosphorylase ab resembles phosphorylase b.