The time course of DNA repair was investigated using the nucleoid sedimentation technique. Human lymphoid cells were irradiated with membrane related equitoxic doses of UVB, UVA/B, UVA and UVA1. Additionally, PUVA treatment of cells using two different light sources was performed. It was found that in the case of UVB and UVA/B irradiation of the cells, DNA repair activity arose lasting not longer than 4 h. However, in the case of UVA and UVA1 no repair activity was detectable with the used method but an increasing nucleoid density believed to indicate apoptosis. This could be shown with the same irradiation protocol. When the irradiation dose of UVA and UVA1 was lowered to one fifth of the equitoxic dose neither repair activity nor apoptosis was detected. There was no difference between UVA and UVA1 with respect to the nucleoid density time course. Hence, the initial absorbing chromophore is probably located in the UVA1 region. In the case of PUVA, we found DNA repair that was complete after 12 h. The UVB part in the irradiation spectrum of a PUVA system had an accelerating effect on the repair kinetics. The time course of DNA density appears to be related to subsequent cell behaviour: apoptosis or incomplete DNA repair including the risk of mutation.