Granulocyte-macrophage progenitor cells (CFU-GM) were found in the intima of human atheromatous aorta. Granulocyte-macrophage colonies were recovered on the 14th day of culturing of intimal cells suspensions on agar. Medium conditioned by normal leukocytes in the presence of phytohaemagglutinin was used as a source of the colony-stimulating factor. Grossly normal and atheromatous intima contained different number of CFU-GM. No GM were recovered from fibrous plaques. By light and electron microscopies, the injured aortic intima contained the clusters of blood-born cells that were at various stages of granulocytopoiesis (including blasts and mature cells) and poorly differentiated lymphocyte-like cells. The results obtained suggest that in human aortic intima proliferation and differentiation of CFU-GM occur at early stages of atherogenesis, prior to fibrous plaque formation.