A factor stimulating a mitogenic activity of peritoneal macrophages is purified from bovine udder. It is identified as a triglyceride, 1-palmitoyl-2-linoleoyl-3-acetyl-rac -glycerol (rac -MADG). In this study, its enantiomers, R-(+)-and S-(-)-1-palmitoyl-2-linoleoyl-3-acetylglycerol (R-(+)-MADG, S-(-)-MADG) are synthesized. Among them, R-(+)-MADG enantiomer turns out to increase a mitogenic activity in mouse peritoneal macrophages. Also, (S)-(-)-MADG shows a low mitogenic activity. Treatment of a macrophage with R-(+)-MADG increases reactive oxygen species(ROS). Furthermore, treatment of macrophages with antioxidant, N-acetyl-L-cysteine (NAC), suppresses the R-(+)-MADG-dependent macrophage proliferation. Results show that the generation of ROS induces in R-(+)-MADG-dependent cell signaling. Treatment of a macrophage with R-(+)-MADG increases the activity of protein kinase C (PKC). Treatment of macrophages with calphostin C inhibits R-(+)-MADG-induced macrophage proliferation. Results suggest that R-(+)-MADG enhances the activity of protein kinase C (PKC) and stimulates the macrophage growth. In conclusions, R-(+)-MADG accelerates the production of ROS and increases the activity of PKC to eventually stimulate macrophage cell growth. The existence of rac -MADG in bovine udder and milk provides passive protection for the neonate and immunostimulatory capabilities.