The pS2 protein belongs to the so called estrogen-dependent proteins, which appear in cells as a result of estrogen stimulation. The present study was aimed at determining prognostic value of estrogen receptor (ER) and pS2 protein expression in mammary cancer cells. The immunocytochemical reactions were performed in paraffin sections of tissue samples from 62 patients with ductal mammary carcinoma using monoclonal antibodies against ER and pS2. The analysis included also survival time of the patients, determined during the five-year observations. The studies documented a correlation between survival time and the level of ER expression (p=0.014) and absence of correlations between survival time and pS2 expression (p=0.55). The results indicate that evaluation of ER expression in mammary cancer cells may assist in defining prognosis of the patients while parallel estimation of pS2 expression in the cells is useless in this respect.