Not every case of mammary carcinoma with expression of estrogen receptors (ER) responds by remission to anti-estrogen treatment. Possible causes of the phenomenon may involve abnormalities of the receptor or defects in mechanisms of signal transmission. One of the ways in which function of the receptor may be detected involves examination of expression of the antigens which appear as a consequence of estrogen stimulation, e.g., expression of pS2 protein. The present study was aimed at comparing ER and pS2 expression in cells of mammary carcinoma, and hence, at finding out whether the presence of ER is equivalent to the sensitivity to estrogen action. In paraffin sections of ductal mammary carcinoma samples obtained from 56 patients, immunocytochemical reactions were performed using monoclonal antibodies against ER and pS2. The results documented positive correlation between the presence of ER and the presence of pS2 in cells of mammary carcinoma (Spearman's rank correlation: r=0.43, p <0.001). Thus, the intensity of pS2 expression was directly related to the expression of ER and the latter was found to be functional.