To examine the capacity of chemical protectors to mitigate damage caused by chronic irradiation by incorporated radionuclides in vitro, cells must be maintained in the presence of the protector during the course of the irradiation. Such long exposures to chemical protectors at concentrations high enough to afford protection usually results in extreme chemotoxicity. To overcome this problem, experimental conditions were developed to allow Chinese hamster V79 cells to be maintained in 5% DMSO for prolonged periods (up to 72 h) with no observable chemotoxicity. Under these conditions, the capacity of DMSO to protect against damage to V79 cells caused by unbound 32P and 3H2O and DNA-incorporated 131IdU, [3H]dThd and 125IdU was examined. The dose modification factors for 32P, 3H2O, 131IdU, [3H]dThd and 125IdU were 2.6 ± 0.5, 2.3 ± 0.3, 1.0 ± 0.1, 1.16 ± 0.07 and 1.07 ± 0.02, respectively. These results show that 5% DMSO is capable of protecting cultured V79 cells against lethal damage caused by β particles emitted by unbound 32P and 3H2O, whereas little or no protection is afforded against damage caused by β particles emitted by DNA-incorporated 131I and 3H or low-energy Auger electrons emitted by DNA-incorporated 1251I.