Monocyte/macrophages play an important role in the development of atherosclerosis. Electron microscopic evidence suggests that in the early stages, lipid laden monocytes leave the lesion to reenter the circulation. This reverse monocyte traffic ceases as the lesion develops. We hypothesize that monocyte/macrophages may not be able to exit the lesion and reenter the circulation because of the reduced expression of CD11/CD18 integrins. We have compared CD11b and CD18 expression of peripheral blood monocytes from normal rabbits (NMø) to atherosclerotic lesion-derived macrophages (AthMø) by anti-CD11b and anti-CD 18 antibody staining, followed by flow cytometry and immunohistochemical staining. AthMø were isolated from aortic lesions of rabbits fed 2 per cent cholesterol diet following balloon angioplasty. AthMø were separated into two regions based on their size and granularity by flow cytometry. All macrophages stained positively with RAM 11. Our results indicated that NMø showed a strong cell surface expression of CD11b and CD18. The less granular and smaller AthMø showed little anti-CD11b or anti-CD18 antibody staining, indicating very little CD11b or CD18 antibody staining, indicating very little CD11b or CD18 expression. The more granular and larger cells showed surface expression of both CD11b and CD18. With respect to CD18, over 90 per cent of NMø expressed CD18, only 37 per cent of the large granular AthMø and less than 1 per cent of the smaller, less granular AthMø stained positive for CD18. Immunohistochemical studies revealed strong surface expression of CD11b and CD18 on normal monocytes.(ABSTRACT TRUNCATED AT 250 WORDS)