Murine receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF) and Multi-CSF (interleukin-3) can exist in both high- and low-affinity forms and demonstrate trans-modulation by several different ligands. In contrast the recently cloned human GM-CSF receptor and murine interleukin-3 (IL-3) receptor display only low-affinity binding. To begin to understand the molecular basis of the formation of high- and low-affinity receptors and their trans-modulation we have developed methods for the solubilization and assay of GM-CSF and interleukin-3 receptors so that their binding characteristics can be studied in cell-free solution. Both receptors displayed a single class of high-affinity binding on intact FDC-P1 cells and IL-3 receptors had unaltered binding characteristics in cells, membranes and in detergent solution. However, GM-CSF receptors were converted to a single class of low-affinity binding in detergent solution while both high- and low-affinity forms were evident in membranes. The basis of affinity conversion of GM-CSF receptors was exclusively a change in the kinetic dissociation rate of ligand. Cross-linking experiments suggested that high-affinity receptors for GM-CSF and IL-3 might consist of two different protein species and, if this is so, the data suggest that this association is more stable for IL-3 than for GM-CSF receptors.