To study if short-term exposure (2h and 6h) of endometrial/endometriotic tissues and cells to 10% seminal plasma (SP) can induce EMT/metaplasia. Basic research experimental study was carried out in a University hospital-based fertility center. Semen samples, peritoneal fluid (PF) from endometriosis patients, endometrial biopsy from premenopausal women, immortalized endometriotic epithelial cell line (12Z), and immortalized endometrial stromal cell line (St-T1b) were studied. Rapid stain identification test (RSID), TGFβ1 immunofluorescence of washed sperms, TGFβ1-ELISA of SP and PF, in vitro study (2h and 6h incubation) and real-time PCR of endometrial tissue and cell lines to analyze gene expression of EMT/metaplasia markers and mediators were done. SP is still detectable in washed semen. TGFβ1 was expressed on the plasma membrane of the sperms and was significantly more concentrated in SP (88.17ng/ml) than PF. 10% SP induced an up-regulation of alpha smooth muscle actin expression in endometrial tissue (p = 0.008) and in 12Z cells (p = 0.05), mostly TGFβ1-independent. TWIST expression was persistently significantly down-regulated while Snail1 and 2 were up-regulated, though insignificant. Our results provide novel evidence to support that even in semen washed twice, SP is still detectable. The changes in EMT/metaplasia markers and mediators give a new insight into a possible effect of SP on the pathogenesis of endometriosis.