To get further insight into the causes of differences between rat splenic and thymic cells with regard to DNA repair synthesis, scheduled (SDS) and unscheduled (UDS) DNA synthesis as well as nucleoid sedimentation of the cells were investigated under the influence of ethidium bromide (EB, 1-1000 micrograms/ml). - At concentrations of greater than or equal to 25 micrograms/ml, EB inhibited SDS of both cell species and UDS of thymic cells; much higher additions of the drug (greater than 200 micrograms/ml) were needed to diminish UDS of splenic cells, lower EB-concentrations (25-175 micrograms/ml) stimulating the UDS of the splenic cell preparation. - The sedimentation rate of splenic and thymic cell nucleoids within neutral sucrose gradients had a biphasic dependence on the EB-concentrations. As compared to thymic cells however, preincubation of splenic cells with 50-250 micrograms EB/ml resulted in a significant greater (15-30 percent) sedimentation distance. - The results suggest that a relationship exists between the stimulation of UDS and the ability of cells to establish a greater DNA compactness in the presence of EB.