Sperm protein 17 (Sp17) is an antigenic protein highly expressed in spermatozoa. Sp17 expression was demonstrated recently in multiple myeloma, suggesting that it may be a novel cancer-testis antigen. Expression of Sp17 mRNA and protein was examined in human ovarian tumors. Sp17 mRNA was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis of RNA derived from epithelial ovarian tumors and normal tissues. RT-PCR analysis detected Sp17 transcripts in 15 of 18 (83%) primary ovarian tumors. The transcript was not detected in RNA derived from normal uterus or cervix, whereas weak expression was noted in some normal ovarian tissue samples. Northern blot analysis showed no detectable Sp17 mRNA expression in normal tissues, including normal ovary, but showed Sp17 expression in 17 of 25 ovarian tumors (68%). To evaluate protein expression, mouse monoclonal antibodies were produced against recombinant Sp17 protein and used in Western blot and immunohistochemical analyses of normal reproductive tissue and primary ovarian tumor samples. Sp17 protein was detected by Western blot analysis in normal spermatozoa and in 8 of 19 ovarian tumor samples. Immunohistochemical studies showed Sp17 expression in spermatozoa, ciliated cells of the female reproductive tract, and most ovarian tumors evaluated. Tumors showed a predominately nuclear localization of Sp17 expression, with some cytoplasmic staining. These results demonstrate that Sp17, a protein with restricted expression in somatic tissues, is expressed in ovarian tumors. Because Sp17 is immunogenic, it may represent a novel target for immunotherapeutic interventions for ovarian cancer patients. © 2003 Wiley-Liss, Inc.