OBJECTIVE: To develop a protocol for immunohistochemistry (IHC) to determine the Schlafen 11 (SLFN11) expression in formalinfixed, paraffinembedded (FFPE) highgrade serous ovarian carcinoma (HGSOC) samples. STUDY DESIGN: We validated a reliable SLFN11 antibody (SLFN11Ab) in IHC choosing between 2 antiSLFN11Abs used for western blot through the development of a SLFN11IHC protocol in culture cell block (CCB) of ovarian carcinoma and in an HGSOC tissue microarray (TMA) series. We applied our protocol to a case series of HGSOC samples to confirm our preliminary results. For each case we evaluated both the intensity score (IS) and the distribution score (DS). A final histological score (HS) was obtained as follows: HS=IS×DS. RESULTS: In CCB and TMA series, the antibody #1 at 1:100 dilution was more reliable. This last one was used in our case series to confirm our IHC protocol. DS showed the following results: 27 cases were no stain, 11 cases showed staining for SLFN11 in <10% of cells, 16 cases showed staining in 10–40% of cells, and the remaining 15 cases showed stain in >40% of cells. IS showed the following results: 25 cases were no stain, in 19 cases a dominant weak stain, in 14 cases a moderate stain, and in 11 cases a strong stain. HS for SLFN11 expression presents a normal distribution with a prevalent (≈60%) intermediate expression. CONCLUSION: We develop a reproducible and standardized IHC protocol to determinate the SLFN11 expression in FFPE HGSOC samples using a modified western blot antiSLFN11Ab. © Science Printers and Publishers, Inc.