The structurally related T cell surface molecules CD28 and CTLA-4 interact with cell surface ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells (APC) and modulate T cell antigen recognition. Preliminary reports have suggested that CD80 binds CTLA-4 and CD28 with affinities (K_d values ∼12 and ∼200 nM, respectively) that are high when compared with other molecular interactions that contribute to T cell–APC recognition. In the present study, we use surface plasmon resonance to measure the affinity and kinetics of CD80 binding to CD28 and CTLA-4. At 37°C, soluble recombinant CD80 bound to CTLA-4 and CD28 with K_d values of 0.42 and 4 μM, respectively. Kinetic analysis indicated that these low affinities were the result of very fast dissociation rate constants (k_off); sCD80 dissociated from CD28 and CTLA-4 with k_off values of ⩾1.6 and ⩾0.43 s⁻¹, respectively. Such rapid binding kinetics have also been reported for the T cell adhesion molecule CD2 and may be necessary to accommodate dynamic T cell–APC contacts and to facilitate scanning of APC for antigen.