Serum is known to inhibit the merocyanine 540 (MC540)-sensitized photoinactivation of cells and enveloped viruses in a concentration-dependent manner. In diagnostic applications of MC540, a moderate amount of serum or serum albumin is frequently added to the staining solution because it enhances the contrast between intensely staining cells (e.g., electrically excitable cells or leukemia cells) and cells with a lower affinity for the dye (e.g., nonexcitable cells, red cells, normal leukocytes). In this communication we report on a quantitative analysis of the interactions of MC540 with serum and serum components. Human serum inhibited the MC540-sensitized photoinactivation of K562 leukemia cells most effectively, followed in order of decreasing potency by calf, newborn calf, horse, and fetal bovine serum. The photoprotective capacity of these five sera was directly proportional to their albumin content. Gel filtration experiments and differential spectroscopy showed that MC540 bound to serum albumin and lipoproteins. Both delipidated and lipidated albumin were capable of binding MC540. However, lipidated albumin had a considerably higher binding capacity and affinity for dye molecules.