[en] The DNA synthesized shortly after ultraviolet (uv) irradiation of Potorous tridactylis (PtK) cells sediments more slowly in alkali than that made by nonirradiated cells. The size of the single-strand segments is approximately equal to the average distance between 1 or 2 cyclobutyl pyrimidine dimers in the parental DNA. These data support the notion that dimers are the photoproducts which interrupt normal DNA replication. Upon incubation of irradiated cells the small segments are enlarged to form high molecular weight DNA as in nonirradiated cells. DNA synthesized at long times (approximately 24 h) after irradiation is made in segments approximately equal to those synthesized by nonirradiated cells, although only 10 to 15 percent of the dimers have been removed by excision repair. These data imply that dimers are not the lesions which initially interrupt normal DNA replication in irradiated cells. In an attempt to resolve these conflicting interpretations, PtK cells were exposed to photoreactivating light after irradiation and before pulse-labeling. After 1 h of exposure approximately 35 percent of the pyrimidine dimers have been monomerized, and the reduction in the percentage of dimers correlates with an increased size for the DNA synthesized by irradiated cells. However, at 24 h after irradiation and 1 h of photoreactivation there are no endonuclease-sensitive sites, even though approximately 50 percent of the pyrimidine dimers remain in the DNA. These data indicate that not all pyrimidine dimers are accessible to the repair endonuclease. (U.S.)