In human WI-38 (diploid) and HeLa S3 (aneuploid) cells, DNA that has undergone repair replication, as a result of UV-induced damage, can later participate in semiconservative synthesis. In WI-38, the fraction of DNA that does so is inversely proportional to the exposure to UV. In HeLa S3, however, no such simple relationship holds; about two-thirds of the repaired DNA replicates normally irrespective of exposure up to <tex-math>$100\ {\rm ergs}/{\rm mm}^{2}$</tex-math>, with lower fractions doing so after higher exposures. There seem to be two kinds of "repaired" molecules after UV-irradiation of HeLa S3: one in which the substituted bases occur in long runs; the other, in short runs only. Using cells synchronized by mitotic selection, evidence is presented for two rounds of semiconservative replication of repaired HeLa DNA.

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