The binding of the enkephalin dimer [D-Ala2, Leu5-NH-CH2-]2 (DPE2) is characterized by its high affinity for receptors on NG108-15 hybrid cells, the affinity constant K = 4.7 X 10(9) M-1 is up to 8-fold that of monomers (0.6 to 1.0 X 10(9) M-1), and a maximal binding capacity equal to one half that of the monomers. Kinetic studies showed that DPE2 binds with a 2-fold higher rate, k1 = 6.3 X 10(7) M-1min-1, than monomers (2.4 to 3.8 X 10(7) M-1min-1), and dissociates at a slower rate than monomers. Dissociation of DPE2 was consistently bi- or multiphasic but increased about 12% only after 3 hr of dissociation in the presence of a large excess of unlabeled enkephalin. The dissociation kinetics of monomers varied with enkephalin and experimental conditions used. Consistent with the value for the maximal binding capacity, the kinetic studies are interpreted in support of the hypothesis that DPE2 binds by cross-linking two subunits of one receptor.