The contribution of intersubunit interactions to allosterically induced conformational changes in phosphorylase are considered. Phosphorylase a, Pa (phosphorylated at Ser-14), is significantly in the active (R) conformation, while phosphorylase b, Pb (nonphosphorylated), is predominantly in the inactive (T) conformation. The structure of glucose-inhibited (T) Pa has been determined at 2.5-A resolution and atomic coordinates have been measured. These data have been used to calculate the solvent accessible surface area at the subunit interface and map noncovalent interactions between protomers. The subunit contact involves only 6% of the Pa monomer surface, but withdraws an area of 4,600 A2 from solvent. The contact region is confined to the N-terminal (regulatory) domain of the subunit. Half of the residues involved are among the 70 N-terminal peptides. A total of approximately 100 atoms take part in polar or nonpolar contacts of less than 4.0 A with atoms of the symmetry-related monomer. The contact surface surrounds a central cavity at the core of the interface of sufficient volume to accommodate 150–180 solvent molecules. There are four intersubunit salt bridges. Two of these (Arg 10/Asp 32, Ser-14-P/Arg 43) are interactions between the N-terminus of one protomer with an alpha-helix loop segment near the N-terminus of the symmetry-related molecule. These two are relatively solvent accessible. The remainder (Arg 49/Glu 195, Arg 184/Asp 251) are nearer the interface core and are less accessible. The salt bridges at the N-terminus are surrounded by the polar and nonpolar contacts which may contribute to their stability. Analysis of the difference electron density between the isomorphous Pa and Pb crystal structures reveals that the N-terminal 17 residues of Pb are disordered. Pb thus lacks two intermolecular and one intersubunit (Ser-14-P/Arg 69) salt linkage present in Pa. The absence of these interactions in Pb is manifested in the difference in the free energy of T leads to R activation, which is 4 kcal more than that for Pa. Difference Fourier analysis of the T leads to R transition in substrate-activated crystals of Pa suggests that the 70 N-terminal residues undergo a concerted shift towards the molecular core; salt bridges are probably conserved in the transition. It is proposed that the N-terminus, when "activated" by phosphorylation (via a specific kinase) behaves as an intramolecular "effector" of the R state in phosphorylase and serves as the vehicle of homotropic cooperativity between subunits of the dimer.