This paper deals with the study of the cell population in 13 samples of normal human pericardial fluid. Large mononuclear cells (LMC) constituted 74.1 +/- 18.5% of the total cell population. These LMC possess the characteristics of macrophages firm adherence to glass intracytoplasmic presence of vimentin without keratin, ultrastructural observation of a lysosomal apparatus cytoenzymatic activities: acid phosphatases, naphthol AS.D acetate esterase and peroxidases, and phagocytosis of Baker's yeasts. All these data clearly show that macrophages are the main component of the pericardial fluid cell population and can be of great significance in the defense mechanisms and physiology of the pericardial space.