Using a new immunoradiometric assay, a total of 100 cytosols obtained from primary breast tumors were examined for content of pS2, an estrogen-regulated protein. In our series, the median pS2 value was 5 ng/mg protein cytosol which was used as the cut-off. pS2 content was not correlated with menopausal status, tumor size, nodal involvement or tumor proliferative activity expressed as labeling index (tritiated thymidine LI). A positive correlation was found between pS2 and estrogen (ER) and progesterone (PgR) hormone receptors. pS2 in association with ER and PgR seems to identify tumor subgroups with diverse hormone responsiveness. Moreover, in favorable and unfavorable prognostic subgroups, LI and pS2 further emphasize this prognostic diversity.