Proteolytic activity in vascular tissue is necessary for cellular migration, remodelling of extracellular matrix and the development of atherosclerotic lesions. Inflammatory cells, mainly macrophages, are numerous in atherosclerotic plaques and may synthesize and secrete proteolytic enzymes. The principal activator of plasminogen in tissues is urokinase plasminogen activator (u-PA). To determine if an activated phenotype of inflammatory cells colocalizes with local expression of u-PA in atherosclerotic vessels, vascular biopsies from 15 patients with peripheral atherosclerotic disease were analyzed by immunohistochemistry on consecutive sections. Anti-CD68 antibodies were used as markers for macrophages and were positive in 14/15 specimens. Anti-CD25 (interleukin-2 receptor-α) antibodies were used to identify inflammatory cells with an activated phenotype and were positive in 9/14 CD68+ specimens. The same 9 specimens were positive for u-PA. A positive reaction for u-PA was found only in specimens with CD25+ cells. Specimens with positive reactions for all three antibodies were further analyzed with computer-assisted image analysis. The colocalization with u-PA was higher for CD25 compared to CD68 in all specimens. Mean percentage of the u-PA-positive area in regions positive for cellular markers was 52% (SEM 6%) for CD25 and 19% (SEM 5%) for CD68 (p < 0.01). The results indicated that the activation of macrophages in atherosclerotic vessels may modulate local proteolysis and be of importance in plaque development and stability.