The optimal monoclonal antibody to examine steroid hormone receptor status of primary breast carcinoma has yet to be defined. Estrogen receptor status was evaluated in 592 cases using routinely prepared paraffin-embedded tissue samples from primary breast carcinomas with the 1D5 (DAKO, Carpinteria, CA) and 6F11 (Novocastra, Newcastle upon Tyne, England) monoclonal antibodies. The stains were compared, assessing the percentage of positive cells stained and their intensity. They also were examined for nonspecific cytoplasmic staining and fixation artifact. In addition, a cost analysis for their production was performed. Overall, 1D5 and 6F11 showed a 97.5% concordance rate. 6F11 stained a significantly higher percentage of cells (P < .0001), more intensely (P < .0001), with less nonspecific cytoplasmic staining (P < .0001). There was no significant difference in fixation artifact between the 2 clones. The cost of antibody used for preparing a 1D5-stained slide was 86% more than for preparing a 6F11-stained slide (dollars 14.27 vs dollars 7.67).