We determined the amino acid and radiolabel sequences of tryptic [32P]phosphopeptides of the purified human estrogen receptor (hER) from MCF-7 cells and Sf9 cells. Serine 118 was identified as a site that was phosphorylated independently of estradiol-binding in MCF-7 cells. Proline is on the carboxy terminus of serine 118, which suggests that the serine-proline may be a consensus phosphorylation site motif for either the mitogen-activated protein (MAP) kinase or p34cdc2 kinase. MAP kinase selectively phosphorylated the recombinant hER in vitro on serine 118 independent of estradiol-binding, whereas p34cdc2 did not phosphorylate the hER. We demonstrated previously that serine 167 of the hER was phosphorylated in an estradiol-dependent manner. We therefore compared the consequence of hER phosphorylation at serine 118 by MAP kinase and phosphorylation at serine 167 by casein kinase II on the receptor's affinity for specific DNA binding. The binding of the hER to an estrogen response element was not altered by phosphorylation with MAP kinase at serine 118 but was significantly increased when phosphorylated at serine 167 by casein kinase II. These data suggest that phosphorylation of the hER by MAP kinase(s) pathways may influence receptor action by a mechanism other than the estradiol-dependent phosphorylation of hER by casein kinase II.