Murine C 1300 neuroblastoma cells bind with high avidity on their membrane surface the nerve growth factor (NGF), a protein capable of inducing differentiation of sympathetic nerve cells. The total binding capacity of NGF by the cells was quantitatively measured by a radioimmunoassay technique, using 125I-labeled NGF. An average number of about 106 molecules of NGF could be bound, at saturation, by each cell with an average relative association constant of about 107 liters/mol. Using synchronized cells, it was found, however, that either the number of molecules of ligand bound or the avidity of the binding interaction between NGF and cells varied depending upon their growth cycle, the maximal-binding occurring during the G1 and early S phase. Binding of [125I]NGF was suppressed by trypsin treatment of the cells, however new receptor sites were rapidly replaced onto the membrane surface within 1–2 h. Cells exposed to 3 M KCl released into the supernate a protein product exhibiting similar high avidity for NGF. Acrylamide gel electrophoresis suggested a restricted molecular heterogeneity of this product, with a major component in the 52,000 mol wt region. Antibodies made specific to this protein were capable, in the absence of the complement, of inhibiting the binding of [125I]NGF by the cells and in the presence of the complement they killed them
