Long-circulating, tumor-accumulating fluorescent liposomes (DiR-labeled). These fluorescent liposomes passively accumulate in solid tumor tissues for a variety of murine tumor models and are labeled with the fluorescent dye, DiR, making them excellent for in vivo and ex vivo imaging studies in rodents. The liposomes are compatible for use with PerkinElmer In Vivo Imaging Systems (IVIS) equipped with DiR filter set (DiR Absorption/Emission: 748/780; IVIS Filter Set: 710 ex/760 em [1]). PEGylated to improve stability, prolong circulation time and reduce reticuloendothelial system clearance for in vivo applications. The lipid composition is very similar to that of the well-characterized Doxil® formulation.