Progesterone receptors are phosphoproteins, in which phosphorylation has been proposed as a control mechanism for some stages of hormone action. Progesterone administration was shown to increase phosphorylation of the receptor from both cytosol and nuclear extracts of whole cells. We have analyzed the receptor phosphopeptides generated by chemical and proteolytic cleavage to assess the number of phosphorylation sites and their approximate location in the receptor. Progesterone receptor was labeled in situ in the presence or absence of hormone in medium containing [32P] orthophosphate, isolated by immunoprecipitation, and then digested with several proteases. The resulting 32P-labeled peptides were resolved by either two-dimensional electrophoresis:chromatography or by reverse-phase high performance liquid chromatography. Multiple phosphopeptides (3-6) were detected after cleavage with trypsin, chymotrypsin, or V8 protease. Major increases in phosphorylation occurred at existing sites since after hormone treatment no new phosphopeptides were found. Individual phosphopeptides showed variable increases in phosphorylation of 1.5-5-fold. The A and B receptor forms showed identical phosphorylation patterns, indicating similar processing in vivo. The phosphopeptide pattern for receptor in nuclear extracts resembled that of cytosol receptor. Chemical cleavage was used to assess the distribution of phosphorylation sites. Cyanogen bromide produced a large 40-kDa polypeptide which contained all of the phosphorylation sites and comprised the residues 129-449. Hydroxylamine was used to cleave a unique bond, Asn-372-Gly-373, in the 40-kDa polypeptide. All of the phosphorylation sites were located on the amino-terminal side of the cleavage. Thus, all of the phosphorylation sites were localized to a specific region (Met-129 to Asn-372) of the progesterone receptor that does not include either the DNA or steroid binding domains.