We have examined phosphorylation of nerve growth factor (NGF) receptor in cultured sympathetic neurons and PC12 cells. Dissociated rat superior cervical ganglion neurons or PC12 cells were incubated with 32Pi to label cellular phosphoproteins. Membrane proteins were solubilized, and NGF receptor proteins were immunoprecipitated with the monoclonal antibody 192-IgG. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography showed that NGF receptor components of Mr = 80,000 and Mr = 210,000 were phosphorylated. Phosphorylation of neither species was affected by treating the cells with NGF or phorbol 12-myristate 13-acetate. When the 80,000-Da protein was subjected to complete trypsin proteolysis and then analyzed by reverse phase liquid chromatography, two 32P-labeled peptides were resolved. The more hydrophobic peptide accounted for most of the 32P and contained only phosphoserine; the other peptide contained phosphoserine and phosphothreonine. No phosphotyrosine was detected in the receptor proteins. When receptor molecules from nonlabeled PC12 cells were immunoprecipitated and then incubated in vitro with [gamma-32P]ATP and the cAMP-independent protein kinase FA/GSK-3, phosphorylation occurred predominantly on serine and to a lesser extent on threonine. However, the immunoprecipitated receptor proteins neither autophosphorylated nor were they detectably phosphorylated by cAMP-dependent protein kinase, casein kinase II, or protein kinase C (the Ca2+/phospholipid-dependent enzyme). We conclude that binding units of the NGF receptor are phosphorylated constitutively in at least two sites in intact cells and that they can be phosphorylated by FA/GSK-3 in vitro.