The myxoma virus protein Serp-1 is a member of the serine protease inhibitor superfamily. Serp-1 potently inhibits human serum proteases including plasmin, urokinase-type plasminogen activator (uPA), and tissue-type plasminogen activator (tPA). Serp-1 also displays a high antiinflammatory activity, rendering it a promising candidate for antiatherosclerotic therapy. In this study, we have thus examined the effect of Serp-1 on de novo atherosclerotic plaque formation and on advanced lesions. Perivascular collars were placed around carotid arteries of ApoE ^−/− mice to induce atherosclerotic plaques and Serp-1 treatment started at week 1 and week 5 after collar placement. Effects of Serp-1 on de novo atherogenesis were characterized by a significantly lower plaque size than that of control mice (18±5×10 ³ versus 57±12×10 ³ μm ² , respectively; P =0.007). Immunostaining showed a 50% ( P =0.004) decrease in the MOMA-2–stained lesion area of Serp-1–treated mice. Treatment of advanced lesions with Serp-1 resulted in a decrease in plaque size and lumen stenosis ( P =0.028). α-Actin staining of these lesions was significantly increased compared with the control ( P =0.017). In both studies, a higher cellularity of the plaque and increased collagen content was observed in Serp-1–treated mice. In vitro studies showed that Serp-1 induces proliferation and migration of vascular smooth muscle cells. In conclusion, Serp-1 inhibits carotid artery plaque growth and progression in ApoE ^−/− mice. Equally relevant, it enhances cellularity of the plaque core potentially leading to improved plaque stability. The above results indicate that Serp-1 constitutes a promising lead in antiatherosclerotic therapy.