DNA replication and repair has been studied in human cells irradiated with single or split doses of UV at 254nm. A dose of 2 J.m(-2) followed by incubation in medium for periods of 12 to 48 h before a second irradiation with 5 or 10 J.m(-2) actually reduces the amount of DNA replication seen after the latter dose alone. Such a preirradiation did not perceptibly alter the rate of excision repair seen after the latter dose when measured by the incidence of UV endonuclease sensitive sites, 3H-bromodeoxyuridine incorporation, or the incidence of cytosine arabinoside induced breaks. Furthermore, the first two parameters were measured in molecules replicating or remaining unreplicated after the latter irradiation. No difference in repair rates was seen between these two classes of molecules.