Biological and pharmacological effects of psoralens are correlated to their capacity to photoreact with pyrimidine bases of native DNA and in particular to form inter-strand cross-linkages. To determine possible prefenrential sites for the photoreaction the Authors have studied the capacity of psoralen to form complexes and to photoreact with various polynucleotides. The Authors have shown that specific sites exist in DNA for the photochemical interaction with psoralen; these sequences which can be considered as specific receptors for photobiological activity of psoralen are represented by alternate sequences in every strand of A and T, corresponding to poly d(A--T) - poly d(A--T). These receptor sites show a high capacity for intercalation and successive photoreaction with psoralen.