The effect of two psoralens, 8-methoxypsoralen (8MOP) and angelicin (ANG), plus near-UV (PUVA) on the transformation capacity of pH 2.3 plasmid DNA on Escherichia coli was studied. Under identical experimental conditions the 8MOP linking to plasmid DNA drastically decreased its transformation capacity compared with the ANG linking. In the case of 8MOP, the decrease depends on the UV dose, as well as on the molar ratios of psoralen and DNA nucleotides. When the effect of short-wavelength UV (UVB) was tested, the higher the molar ratios, the more the combined effects of PUVA and UVB were negative.