Decay-accelerating factor (DAF) is a 70-kD membrane glycoprotein that regulates autologous complement activation, by preventing assembly of alternative or classical C3/C5 convertases, and has been shown to have a wide tissue distribution. In this study, DAF antigen has been demonstrated at the intercellular spaces of normal human epidermis with monoclonal antibody against DAF using the peroxidase-anti-peroxidase method. The amount of DAF was greater at the granular layer than the basal cell layer as judged by intensity of the staining. Western blot analysis of DAF in the epidermis showed a 55-kD band, whereas that of buffy coat cells was approximately 67 kD. When DAF of the epidermis was treated with neuraminidase, the molecular weight was reduced to 53 kD, whereas that of buffy coat cells was 56 kD. These results indicated that the content of sialic acid of DAF in the epidermis was different from that of buffy coat cells. In phosphatidylinositol-specific phospholipase C (PIPLC)-treated normal human skin, DAF was not demonstrated in the epidermis, whereas DAF remained unchanged on the elastic fibers. After the treatment of the epidermis by PIPLC, DAF was released into the buffer shown by Western blot analysis. These results suggested that DAF on the epidermis was anchored to keratinocyte via phosphatidylinositol (PI), whereas the anchoring mechanism of DAF on the elastic fibers was not through PI.