AbstractThe kinetic parameters involved in determining the steady-state interaction of Multi-CSF with FDCP-1 cells at 37°C have been determined by kinetic analysis under steady-state conditions and by curve-fitting the rate of approach to steady-state conditions. The two methods are in substantial agreement and yield values of Vr=28 receptors/cell/min for the rate of appearance of receptors at the cell surface,keandki=0.061 min−1and 0.0044 min−1for the rate constants of internalization of occupied and unoccupied receptors, respectively,kh=0.008 min−1for the rate constant of degradation of internalized ligand, ka=2.9×108M−1min−1for the rate constant of association andkd=0.11 min−1for the rate constant of dissociation of ligand with receptor. Analysis of steady-state conditions indicated that Multi-CSF caused substantial down-regulation of surface receptors and that considerably more Multi-CSF was inside the cell than at the cell surface. The implications of these results for utilization rates of Multi-CSF by FDCP-1 cells and the relationship of receptor occupancy to biological activity are discussed.