Monocyte/macrophages adhere to cells (lymphocytes, vascular endothelial and other cell types) and to extracellular matrix components (fibronectin and laminin) by using specific cell surface adhesive structures. In the present study we have analyzed expression of integrins, immunoglobulin (Ig)-related, selectins, and other adhesion molecules on blood monocytes, in vitro differentiated macrophages (ivMs), and alveolar macrophages (AMs), obtained from healthy nonsmokers by bronchoalveolar lavage (BAL). We have also investigated expression of adhesion molecules on myelomonocytic cell lines HL-60, THP-1, KG-1, and U937 before and after tetradecanoyl phorbol acetate (TPA)-induced differentiation. With regard to the integrin family, monocytes expressed beta 1 (CD29), alpha 4, alpha 5, alpha 6, beta 2 (CD18), CD11a, CD11b, and CD11c subunits, but not alpha V (CD51). Some reactivity with mAbs against the platelet antigens CD41b (IIb) and CD61 (beta 3) was detected. The Ig-related molecules CD54 (ICAM-1), ICAM-2, and CD58 (LFA-3) were expressed, as well as L-selectin and the carbohydrate ligands Le(x) (CD15) and sialyl Le(x). Immunolabeling for the structurally unrelated molecules CD44 and CD36 was strongly positive. In comparison to monocytes, AMs showed much lower expression of alpha 4, alpha 6, beta 2, CD11a, CD11b, L-selectin, Le(x), and sialyl Le(x). Moreover, ICAM-2 and CD36 were practically absent whereas expression of alpha 3, but not of CD11c, was higher. Similar results were obtained with ivMs. All four myelomonocytic cell lines showed down-regulation of alpha 4 and up-regulation of CD11c after TPA treatment. These findings indicate that maturation of monocytes into macrophages is accompanied by characteristic changes in adhesion molecule expression. The particular array of adhesion molecules on monocytes and macrophages may account for differences in the functional properties of these cells.