The ability of compounds to adsorb passively to hydrophobic polymer surfaces composed of, e.g., polystyrene generally is restricted to limited types of molecules such as proteins. Some proteins, many peptides, polysaccharides, oligonucleotides, and small molecules as well as pro- and eucaryotic cells cannot adsorb directly to such surfaces. Also, solid phase adsorbed antigens, antibodies, or gene probes may not be recognized by its corresponding ligand due to denaturation or steric hindrance of the molecular tertiary structure. Covalent binding, on the other hand, orientates all immobilized compounds in a defined way on the solid phase, thereby exposing the interacting sites on the enzymes, antibodies, gene probes, etc. Here we describe a method for modifying a polymer surface by contacting the polymer with derivatives of psoralen under irradiation with long-wavelength UV light. The psoralen derivatives were immobilized covalently on the polymer surface by this process. The psoralen molecules was conjugated to appropriate chemical linkers, incubated in aqueous solutions, and irradiated with UV light. This resulted in solid phase introduction of functional groups such as, e.g., amino groups on the polystyrene surface. The functional groups could subsequently be used for immobilization of biomolecules using conventional cross-linker technology. The method only involved premodification of the psoralens to be immobilized whereas no pretreatment of the polymer was required. Psoralen modified microtiter plates seems to have future application for the development of solid phase hybridization and immunoassays.