Adjacent frozen sections of 102 consecutive female breast carcinomas were examined for the expression of the Ki-67 antibody-reactive proliferation-associated nuclear antigen and of estrogen and progesterone receptors with the use of monoclonal antibodies and peroxidase histochemistry. The results of steroid receptor stainings were semiquantitatively assessed (histoscore) on the basis of nuclear staining intensity and the percentage of positively stained carcinoma cell nuclei. Carcinomas negative for either receptor had significantly higher percentages of Ki-67-positive cells. The highest percentages of Ki-67-positive cells were observed in carcinomas negative for both estrogen and progesterone receptors. There was a highly significant decrease in receptor histoscores with increasing proliferative cell fractions as determined by Ki-67 positivity. No significant (progesterone receptor) or poor negative correlation (estrogen receptor) was observed when proliferative cell fractions were related to receptor concentrations from conventional steroid-binding assays. Immunoperoxidase staining for the Ki-67 antibody-defined proliferation antigen and steroid receptors in tissue sections provides a simple means to gain information of therapeutic and prognostic importance.