Recent studies have shown both macrophages and lymphocytes in very early intimal lesions of experimental aortic atherosclerosis. The authors obtained fresh samples of human aortic wall, which had been removed in the course of aortocoronary bypass graft surgery. Intimal fatty streaks were identified macroscopically and six were studied immunohistochemically. The fatty streaks contained foam cells that were virtually all labeled by antibodies directed against members of the mononuclear phagocyte series (RFD-2 and RFD-7). Macrophages demonstrated acid phosphatase activity and marked expression of HLA-DR, suggesting activation. Other monoclonal antibodies (UCHT-1, OKT-4, and RFT-8) identified T lymphocytes, of both helper and suppressor phenotypes, within the fatty streaks. T lymphocytes of suppressor phenotype appeared to predominate over helper cells. B lymphocytes were not detected. The presence of activated macrophages and T lymphocytes in the fatty streaks indicates that components of a cell-mediated immune response are present. Such an immune process may be important in the pathogenesis of human atherosclerosis.