The double immunohistochemical detection of estrogen receptor (ER) and the estrogen-regulated 52,000-dalton protein (52kDa-P) was performed on 35 fine needle aspirates of primary breast cancers. The nuclear ER, stained brown, was differentiated from the 52kDa-P, stained in red, in the cytoplasm of the same cells. The significant correlation obtained between the percentages of positive aspirated cells and the cytosolic concentrations of these two markers showed the validity of semiquantitative detection in fine needle aspirates, which appeared to be more representative of the content of these markers in tumors than was the detection in a single tissue section. Tumor heterogeneity also was defined by this double immunostaining, since among the tumors positive for both ER and 52kDa-P (18 of 35 cases), a heterogeneous group of 12 tumors contained the markers in two distinct cell populations, whereas a more homogeneous group of 6 tumors contained double-stained cells. The first information provided by this study is that the 52kDa-P is not correlated with the ER. This absence of correlation is in accordance with other studies and indicates that the 52kDa-P is associated with tumor proliferation and possibly invasion, rather than with hormone responsiveness or differentiation. The feasibility and validity of double immunostaining of these two markers in human breast cancer aspirates should stimulate larger studies--with clinical follow-up of the patients--aimed at establishing the clinical usefulness of this presurgical test.