Monocyte/macrophage differentiation was studied in biopsy samples of multiple sclerosis (MS) lesions obtained in the early course of the disease. Macrophages were identified by immunocytochemistry using a panel of antibodies recognizing different macrophage-activation antigens. The number of cells stained with each antibody was related to the demyelinating activity of the lesions as detected by the presence of myelin degradation products. The pan-macrophage marker Ki-M1P revealed the highest numbers of macrophages in early and late active lesions. Lower numbers were encountered in inactive, demyelinated, or remyelinated lesions. The acute stage inflammatory macrophage markers MRP14 and 27E10 were expressed in either only early active (MRP14) or early and late active (27E10) lesions, thus allowing the identification of actively demyelinating lesions. The chronic stage inflammatory macrophage marker 25F9, in contrast, showed increasing expression with decreasing lesional activity. These findings indicate a differentiated pattern of macrophage activation in MS lesions and allow the staging of demyelinating lesions in routinely fixed and paraffin-embedded tissue.