Immunohistochemical stainings for protectin (CD59) and for the macrophage/monocyte antigen were performed in muscle specimens of polymyositis (PM) and dermatomyositis (DM) patients. In control muscles, a granular immunostaining of CD59 at the surface of 10-15% from among muscle fibers and an intense reaction in endothelia of blood vessels were noted. Strong immunostaining appeared inside vacuoles of vacuolar degeneration changes of muscle fibers, and in regenerating muscle fibers in PM and DM. No macrophages were found next to the CD59 positive muscle regions. The results seem to strongly support a role of the complement (C) system and of the factors protecting cells from the C-induced damage in the mechanism of muscle injury in PM and DM. The findings can have treatment implications.