The ability to specifically image macrophages may enable improved detection and characterization of atherosclerosis. In this study we evaluated the in vitro uptake of gadolinium (Gd)‐containing immunomicelles (micelles linked to macrophage‐specific antibody), micelles, and standard contrast agents by murine macrophages, and sought to determine whether immunomicelles and micelles improve ex vivo imaging of apolipoprotein E knockout (ApoE KO) murine atherosclerosis. Murine RAW 264.7 macrophages were incubated with Gd‐DTPA, micelles, and immunomicelles. Cell pellets were prepared and imaged using a 1.5 T MR system with an inversion recovery spin‐echo sequence to determine the in vitro T₁ values. Ex vivo analysis of mouse aortas was performed using a 9.4T MR system with a high‐spatial‐resolution sequence (78 × 39 × 78 μm³). The T₁ value was significantly decreased in cells treated with micelles compared to Gd‐DTPA (P < 0.0001), and in cells incubated at 4°C with immunomicelles compared to micelles (P < 0.05). Ex vivo MRI signal intensity (SI) was significantly increased by 81% and 20% in aortas incubated with immunomicelles and micelles, respectively. Confocal microscopy demonstrated in vitro and ex vivo uptake of fluorescent immunomicelles by macrophages. Immunomicelles and micelles improve in vitro and ex vivo MR detection of macrophages, and may prove useful in the detection of macrophage‐rich plaques. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.