Coronary patients with unstable angina in the presence of considerably changed serum lipid spectrum showed marked changes in peripheral monocyte metabolism (accumulation of lipids and declining cytoplasmic lysosome levels). These changes were partly induced by pathologic components of blood sera, such as atherogenic lipoproteins and circulating immune complexes, as evidenced by the demonstrated capacity of coronary patients' serum to cause lipid accumulation and a drop of monocyte lysosome level in normal controls. Changes in blood monocyte metabolism of coronary patients may reflect the severity of atherosclerotic developments.