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Background: The aims of this study were to assay the activity of adenosine deaminase (ADA) in estrogen receptor positive (MCF-7) and negative (MDA-MB468) breast cancer cell lines. Methods: MDA-MB468 and MCF-7 breast cancer cell lines were cultured in complete medium, striped serum with and without 0.0 1~-LM diethylstilbestrol (DES), complete medium in the presence and absence of 111M tamoxifen for 20 hr. Adenosine deaminase activity was determined using the colorimetric method described by Guisti and Galanti. Results: It was found that the activity of enzyme in estrogen receptor positive (ER+) cell line (MCF-7) was significantly higher than that of estrogen receptor negative breast cancer cell line (MDA-MB468). ADA activity in MCF-7 cells cultured in the presence of tamoxifen or charcoal-striped serum was significantly lower than that of control. Furthermore addition of diethylstilbestrol (DES) to the striped serum increased the value of ADA activity to that of control. Unlike MCF-7 cells, the activity of ADA in MDA-MB468 cells remained unchanged upon treatment with tamoxifen or striped serum. Conclusion: These findings suggest estrogen responsiveness of ADA expression in MCF-7 cells.