A three-dimensional tissue culture model was used to investigate the biological effects of nonuniform distributions of DNA-incorporated (125)I in mammalian cells. Chinese hamster V79 cells were labeled with (125)I-iododeoxyuridine, mixed with unlabeled cells, and multicellular clusters ( approximately 1.7 mm in diameter) were formed by gentle centrifugation. The highly localized energy deposition caused by (125)I decays results in very high equivalent doses delivered to the labeled cells and low equivalent doses delivered to the unlabeled cells. The clusters were assembled and then maintained at 10.5 degrees C for 72 h to allow (125)I decays to accumulate, dismantled, and the cells were plated for colony formation. When 100% of the cells were labeled, the survival fraction was exponentially dependent on the mean radioactivity per labeled cell. A two-component exponential response was observed when either 50 or 10% of the cells were labeled. These experimental data, coupled with theoretical dosimetry calculations, indicate that bystander effects play an important role in the killing of unlabeled cells when nonuniform distributions of DNA-incorporated (125)I are present.