Measurement of colony forming ability following exposure to gamma-rays has been performed on human retinoblasts transformed with either adenovirus 5 or 12 early region 1 DNA, adenovirus early region 1A plus activated N- or H-ras DNA or SV40 DNA. In contrast to recently reported results (M.D. Sklar, 1988, Science, 239, 645-647), we found no general correlation between transformation with activated ras and increased radiation resistance. Similarly, there was no correlation between D0 values and the level of expression of ras p21 in transformed human retinoblasts as determined by liquid competition assay. Indeed, cell lines with very similar D0 values had ras contents varying by up to one hundred fold. Cell lines transformed with SV40 DNA were generally less sensitive to ionising radiation than adenovirus and/or ras transformants, but even so the variation in sensitivity within these encompassed the whole spectrum of values obtained for the ras transformants. It may be interesting to note, however, that two out of the three ras transformants which were least sensitive to gamma-rays were cell lines expressing the highest levels of p21.