Several types of solid tumors are known to accumulate relatively large amounts of photosensitizing compounds and to retain them for prolonged periods. The addition of aqueous solutions of moderately or strongly hydrophobic dyes to serum results in a highly preferential association of the dye with the three main lipoprotein families, namely high-, low- (LDL), and very low- density lipoproteins. The photosensitizer molecules bound to the various lipoprotein fractions have a different fate, both in the rate of clearance from the serum and on the biodistribution in the organism and in tumor tissues. One component of the lipoprotein class, the LDL, exhibits a preferential interaction with cells having a high mitotic index, including tumor cells; the process involves, at least in part, a receptor-mediated endocytosis. Several lines of evidence indicate that the transport of photosensitizers by lipoproteins, and in particular by LDL, is a major determinant of the efficiency and selectivity of tumor targeting.