For the purpose of developing a cell-free binding assay suitable for the large scale screening of receptor agonists and antagonists, the extracellular ligand binding domains of the low affinity NGF receptor (p75 (5^{\mathrm{NGFF}}) ), a splicing variant of the fibroblast growth factor receptor (FGFR), and the PDGF (\beta)-receptor (PDGFR) were modified by the biotinylation of amino groups. After immobilization in streptavidin-coated microtiter wells, it was determined by ligand binding analysis that all three receptors retained high affinities (0.7-1.8 nM) for their respective ligands. Two receptors, p75 ({ }^{\mathrm{NGFR}}) and FGFR, also displayed low affinity ligand binding ( (45-85 \mathrm{nM}) ) which is likely a result of differential biotinylation. The three assay systems displayed excellent tolerance to several solution variables including organic solvents and high salt, with the exception of the FGF assay's sensitivity to thiols. The consistency obtained by using a coated well protocol, coupled with elimination of cell culture while maintaining natural binding affinities, demonstrates that the immobilized biotinylated extracellular domain creates an excellent system for pharmaceutical screening. (c 1993 Academic Press, Inc.