Photoaffinity labeling with 8-N3[32P]cAMP and in vivo phosphorylation with 32PO4 provide sensitive methods for the identification of the surface cAMP receptor. Photoaffinity labeling achieves high specificity while in practice, the in vivo32P labeling affords considerably higher sensitivity. Both methods allow fairly direct assays of cellular responses to an extracellular cAMP stimulus at the level of the receptor—a shift in electrophoretic mobility and a rise in phosphorylation. Quantification of the shift in mobility, assayed by photolabeling, has shown a strong correlation between modification of the receptor and adaptation. These labelling techniques will also help to elucidate the role of the concomidant changes in receptor phosphorylation, which may cause the change in electrophoretic mobility. Furthermore, photolabeling and 32P labeling will allow study of the possible multiple affinity classes of the cAMP receptor and of the possible expression of cAMP receptors in later stages of development. © 1988, Elsevier Inc. All rights reserved.