The platelet derived growth factor (PDGF) plays an important role for development of atherosclerosis. We therefore immunostained carotid atheroma specimens for PDGF. We also detected dividing cell species of the atheroma with in vitro labeling of bromodeoxyuridine (BUdR). Thirty specimens of carotid atheroma were obtained by end arte recto my and they were incubated for 3 hours with Dulbecco's modified Eagle medium/20% fetal calf serum culture medium containing BUdR/fluorodeoxyuridine (FUdR). They were ethanol-fixed[ thin-slicedand immunostained for BUdR, PDGF, smooth muscle actin and macrophage. The PDGF immunoreactivity was mainly detected in the macrophages of the subendothelial area, where BUdR-positive cells were present. Percentage of BUdR-positive cells in the atheroma specimens ranged from 3% to 15%. The BUdR-labeled small cells were mainly located in the subendothelial area, and they were identified as non-foamy macrophages by double immunostaining with anti-macrophage antibody. The results indicate that nonfoamy macrophages have potentials for cell division and they might play an important role for the development and growth of atheroma by secreting PDGF. [Neurol Res 1995; 17: 345-348]