Leukemia inhibitory factor (LIF) is a pleiotropic cytokine whose activities appear to be mediated through a single heterodimeric receptor complex. Human LIF (hLIF) can bind to and activate mouse LIF (mLIF) receptors but mLIF is unable to bind to hLIF receptors. Cross-species competition of mLIF and hLIF for binding to the mLIF receptor was found to be dependent on which ligand was used as the radioactive tracer (Layton, M. J., Cross, B. A., Metcalf, D., Ward, L. D., Simpson, R. J., and Nicola, N. A. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 8616-8620), and this phenomenon was investigated in the present study. We found that hLIF bound to the low affinity mLIF receptor with a 100-500-fold higher primary affinity and lower kinetic dissociation rate than mLIF, but both ligands displayed a single rate of ligand dissociation. In contrast, the binding of hLIF to low and high affinity hLIF receptors revealed two classes of binding site. The observed tracer-dependent phenomena suggested that both mLIF and hLIF interfere with the binding of each other to the mLIF receptor. A model is presented in which hLIF binds to two sites on mLIF and hLIF receptors, one of which interferes with the common site for mLIF. This model may reconcile some of the observed complexities of LIF/LIF receptor interactions.