It is generally accepted that the cell population of naturally occurring and experimental atherosclerotic lesions is constituted by smooth muscle cells and non-myogenic foam cells of monocytic origin. In the present investigation we studied aortic fatty streaks from cholesterol-fed African green monkeys. In addition to the traditionally recognized cell types, the majority of the lesions examined contained intimal granulocytic cells identified by their ultrastructural characteristics and granular content as neutrophils, mast cells-basophils, and eosinophils. The neutrophils, and mast cells-basophils additionally contained numerous cytoplasmic lipid droplets. The consistent observation of these cell types in our material suggests that these granulocytic elements are part of the cell population of fatty streaks. The role of these cells is not clear as yet, but it is likely that the enzymatic activity of neutrophils such as lipase, phospholipases A and B, elastase and collagenase may play a role in the clearing of arterial lipid as well as in arterial wall remodeling. The content and release of heparin and histamine by basophils and mast cells may play a role in preventing thrombus formation and in promoting lipolysis. Eosinophil peroxidase may activate histamine release by basophils and mast cells. The cytoplasmic lipid accumulation by neutrophils, basophils and mast cells may in turn contribute to the population of foam cells in these lesions.