Treatment of rabbit mammary prolactin receptor with N-acetylimidazole resulted in loss of prolactin binding activity. Loss of activity of the particulate receptor was time and concentration dependent with 100 mM reagent causing total inactivation in 10 min. Similar results were obtained with solubilized receptor preparations, but at lower reagent concentrations. The loss of binding activity was due to loss in the number of binding sites. Incubation of the reagent inactivated membranes or soluble receptor with hydroxylamine for 3 hr resulted in 80-90% reactivation of the prolactin binding activity. These results indicate the possible involvement of tyrosyl residue(s) on the receptor in the prolactin-receptor interaction.