The binding and uptake of the colony-stimulating factor CSF-1 by peritoneal exudate macrophages (PEM) from lipopolysaccharide insensitive C3HlHe.l mice was examined at 2"C, and at 37Β°C. At 2"C, '251-CSF-1 was bound irreversibly to the cell surface. At 37"C, 90% of the cell surface associated '251-CSF-1 was rapidly internalized and subsequently degraded and the remaining 10 % dissociated as intact '251-CSF-1. Thus classical equilibrium or steady state methods could not be used to quantitatively analyze ligand-cell interactions at either temperature, and alternative approaches were developed. At 2"C, the equilibrium constant (& < 10-l3M) was derived from estimates of the rate constants for the binding (16" = 8 x ldM-'s-') and dissociation (bff < 2 X IO-'s-') reactions. At 37"C, the processes of dissociation and internalization of bound ligand were kinetically competitive, and the data was formally treated as a system of competing first order reactions, yielding first order rate constants for dissociation, bff = 0.7 min-' (t1,2 = 10 min) and internalization, kin = 0.07 min-' (t1/2 = 1 min). Approximately 15 min after internalization, low-molecular weight 1251-labeled degradation products began to appear in the medium. Release of this degraded '251-CSF-1 was kinetically first order over three half-lives (& = 4.3 x min-', t112 = 16 min). Thus CSF-1 binds to a single class of receptors on PEM, is internalized with a single rate limiting step, and is rapidly destroyed without segregation into more slowly degrading intracellular compartments.