Local estrogen production by aromatase plays a more important role than circulating estradiol in breast tumor promotion. Recently, it has been reported that the activity of the aromatase enzyme is tightly regulated by posttranscriptional modifications, such as phosphorylation. Estrogen stimulation of hormone-dependent breast cancer cells rapidly enhances aromatase enzymatic activity through an increase of tyrosine protein phosphorylation. Here, we describe a novel assay protocol to detect changes in the phosphorylation status of the purified six tandem histidine-tagged form of human aromatase transiently expressed in breast cancer cells. This method overcomes the limitations of current immunoprecipitation techniques performed using commercial available aromatase antibodies.