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The incorporation of proto‐, uro‐ and hematoporphyrin in low density lipoproteins (LDL) of human blood has been studied by equilibrium dialysis, fluorescence and absorption spectroscopy. The lipoproteins may efficiently compete with albumin in the binding of protoporphyrin to human blood proteins in patients suffering from protoporphyria. It can be concluded that hydrophobic porphyrins bind to blood proteins. The complexation of hydrophobic porphyrins in LDL is responsible not only for efficient photodynamic effect at the lipoprotein level, but also for photoinduced lipid peroxidation and for consumption of β‐carotene incorporated into LDL which are one of their natural carriers. The water‐soluble uroporphyrin, although an efficient photosensitizer for the LDL apoprotein photoinactivation, is much less efficient for lipid peroxidation and β‐carotene bleaching. The 353 nm laser flash photolysis shows that porphyrin triplet states are not affected by the physiological β‐carotene content of LDL but are fully accessible to oxygen.