AbstractBecause paraffin embedded tissues of cases that were reactive to the antibody for estrogen receptor in the Abbott ERICA kit stored at room temperature (RT) as 3–5μm sections on glass slides lost reactivity, freshly cut sections were retested for antigen-antibody reaction to assure reactivity was not lost when tissues were stored at RT in the block. In an attempt to identify the time frame in which reactivity was lost as well as identify a better slide storage method, 4 methods were studied: RT; 4°C; deparaffinization and storage in 10% sucrose in PBS at 4°C; and deparaffinization and storage in Abbott storage medium at 4°C. Methods were assessed at 1 wk, 2 wk, 4 wk, 8 wk, and4 mo. Slides archived for at least 10 mo at RT were also studied. Antigen reactivity was scored by intensity of stain and percent of cells positive. Storage at RT produced the most degradation of intensity over the first 4 wk and significant loss of stain in the archival material. Slides stored in sucrose or Abbott storage medium showed the most preservation at 4 wk. All methods showed continued degradation of staining beyond 4 wk, although sucrose storage had the least alteration. Sucrose storage also showed improvement of staining quality even after short periods of time. (The J Histotechnol17:115, 1994)