Proliferative activity (expressed as 3H-thymidine labeling index, 3H-TdR LI) was evaluated on a series of 281 primary tumors recruited in two years in 6 different institutions from central Italy. 3H-TdR LI proved to be low in intraductal, or well and moderately differentiated, or hormone receptor positive tumors. Conversely, no relation was observed between 3H-TdR LI and menopause, tumor size, or lymph node involvement. An inverse relation was observed between 3H-TdR LI and hormone receptor content. Specific patterns of 3H-TdR LI value and ER content association were observed as a function of menopause, lymph nodal status, and degree of lymph nodal involvement.