The extent of association between donor transforming deoxyribonucleic acid (DNA) and recipient DNA in Haemophilus influenzae as a function of ultraviolet (UV) dose to the transforming DNA has been measured by isopycnic analysis of lysates of (3)H-labeled recipient cells exposed to DNA labeled with (32)P and heavy isotopes. Except for doses above 15,000 ergs/mm(2), the results of these measurements are in good agreement with previous estimates made by another technique. Experiments with a mutant temperature sensitive for DNA synthesis and another mutant defective in excision of pyrimidine dimers suggest that the discrepancy between the methods of high doses results from DNA synthesis, in which portions of the associated donor DNA containing pyrimidine dimers are excised and broken down, and the components are reutilized for synthesis. Repair of UV-irradiated, transforming DNA during incubation of recipient cells is observed as an increase in transforming ability when fractions from CsCl gradients of cell lysates are assayed on excision-deficient cells. When transforming DNA containing markers of different UV sensitivities is used, repair of the UV-resistant nov marker by excision proficient cells takes place exclusively in the donor DNA that is associated with recipient DNA, and this repair is observed even in the absence of DNA synthesis. However, no repair is observed in the case of the more UV-sensitive str marker, possibly because excision events may remove a large fraction of the integrated str markers in addition to repairing a small fraction of the integrated DNA containing this marker.