In this study we show that SV-IV, a major immunomodulatory, anti-inflammatory, and sperm immunoprotective protein secreted from the rat seminal vesicle epithelium, acts in vitro as a substrate of protein kinase C (PKC) competing efficiently with H1 histone, a very well known PKC substrate. Electrospray mass spectrometry (ES-MS) analysis demonstrated that approximately 10% of the native SV-IV molecules were phosphorylated by PKC and that such a modification involved only a single serine residue (Ser58) out of the 22 occurring in the protein. Interestingly, this modification produced a substantial enhancement (approximately 50%) of the native SV-IV's ability to stimulate the activity of both horseradish peroxidase (POD) and selenium-dependent glutathione peroxidase (GPX), an enzyme that is known to protect the mammalian spermatozoa from oxidative stress and loss of motility in the female genital tract following ejaculation. In contrast, the phosphorylation of SV-IV on Ser58 did not produce any effect on the anti-inflammatory properties of SV-IV, as measured by its ability to inhibit the phospholipase A2.