Equilibrium constants for the binding of protoporphyrin to serum albumin and hemopexin and liver cytosolic fatty acid-binding protein of the rat were determined fluorometrically. The experimental equilibrium constant [106M−1 (mean ± S. D.)] values were 8.4 ± 1.3, 10.0 ± 2.4 and 34.0 ± 3.0 for albumin, hemopexin and liver fatty acid-binding protein, respectively. Statistical analysis showed the equilibrium constant of binding of protoporphyrin to liver fatty acid-binding protein to be significantly (p < 0.01) higher than that to albumin and hemopexin. The data suggest that in patients with erythropoietic protoporphyria an equilibrium gradient may exist which favors the uptake by hepatocytes of plasma protoporphyrin as a result of its greater affinity for intracellular liver fatty acid-binding protein.