Co-cultured C3H 10T1/2 cells, in which transformed cells were grown as discrete colonies on top of density-inhibited monolayers of untransformed cells, were used to determine the potential usefulness of a short term assay system for the study of differential radiation effects as they may apply to cell populations with differing turnover rates, but in close physical contact. Mixed cultures were exposed to either an acute dose of 20 Gy of Cs-137 gamma rays, or a dose of 72 Gy delivered at a low dose rate of 0.34 Gy per hour. These treatments resulted in approximately equal levels of damage to the untransformed monolayers. At 10-day intervals after treatment, representative flasks from each dose group were examined for evidence of degeneration, and subsequent regrowth, if any, of the transformed colonies and untransformed monolayers. For comparable amounts of visible damage to the untransformed monolayers, the low dose rate irradiation was more effective at delaying regrowth of, or even eradicating, transformed colonies. These results are consistent with expectations based on previous results in which dose-rate or dose fractionation isoeffect curves were compared for these two cell types, grown independently in plateau phase cultures.