Binding of the two photosensitizers, 8-methoxypsoralen (8-MOP) and 5-methoxypsoralen (5-MOP), to serum proteins and to epidermis was measured. 8-MOP binds to serum proteins with an apparent dissociation constant (Kd) of 4 x 10(-5) M. Under conditions of oral therapy, serum concentrations of the photosensitizer 2 h after administration are usually in the range of 100-1000 ng per ml serum. In this concentration range, 75-80% of the drug was found to be reversibly bound to serum proteins. 5-MOP shows a higher binding affinity to serum proteins and 98-99% of the drug is protein bound. The binding of both psoralen derivatives appears to take place mainly to serum albumin. 5-MOP and 8-MOP bind to different and non-interacting sites on serum proteins and the binding of the one has no effect on the binding of the other methoxypsoralen. Both photosensitizers bind reversibly to human epidermis. 8-MOP concentration in the epidermis is increased by ten to twenty fold compared with the equilibrium buffer. 5-MOP shows a higher binding affinity, resulting in a higher tissue concentration of the photosensitizer. As in serum, the two drugs appear to be bound in the epidermis to independent and non-interacting sites. No binding competition was found between the two methoxypsoralens and hydrocortisone, fluocinonide and acetyl salicylic acid, either in serum or in epidermis, using up to 1000 fold higher concentrations as compared with those of 5-MOP and 8-MOP.