AbstractImmunohistochemical staining for the estrogen receptor can be influenced by a variety of technical variables, including degradation of staining with storage of tissue sections on glass slides. The epitope of the commercially available rat monoclonal antibody (H222, ER-ICA) has demonstrated a significant loss of immunoreactivity in tissue sections stored for 8 weeks. The purpose of the present investigation was to determine if the ERlD5 antibody is associated with a similar loss of reactivity. Tissue sections were stored on glass slides at room temperature for short (1–2 days) or long (>365 days) intervals prior to immunostaining with the ER1D5 antibody. Evaluation of immunostaining was performed by manual (microscopic assessment) and automated (image cytometry) methods. No difference in the intensity of the ER1D5 immunoreaction or the number of cells staining was observed by visual inspection. Although the mean summed optical density of ERlD5 staining was less in sections stored for long intervals relative to short intervals, this difference was not statistically significant. These results suggest that the ERlD5 antibody is suitable for staining tissue sections stored on glass slides at room temperature for as long as 1 year. (The J Histotechnol19:23, 1996)