Seven cell lines were analyzed for clonogenic survival after irradiation with photons (60Co) or accelerated helium or nitrogen ions. The cell lines showed different sensitivity to photon radiation and most of the differences decreased after irradiation with helium ions with a linear energy transfer (LET) of about 40 keV/microns. However, all cell types had individual LET sensitization patterns and the mean relative biological effectiveness (RBE) at 10% survival ranged from 1.46 +/- 0.12 to 2.41 +/- 0.26 for the helium ions. This difference was significant and the differences increased further when higher survival levels were considered. There was only a weak tendency towards a relation between photon and helium ion sensitivity when the linear component of the survival curves, the alpha-values, were compared, and no relation at all for other parameters. It was not possible to predict the response to an increased LET from the photon responses obtained. Three of the cell lines were also irradiated with nitrogen ions with an LET of 125 keV/microns. These cells were, as expected, sensitized further and the average RBE at 10% survival was 3.67 +/- 0.67. However, one cell line was more resistant than the others in this case. Furthermore, the quadratic component of the survival curves, the beta values, were higher after irradiation with nitrogen than with helium ions. Thus, several irregular and unexpected results were seen when the LET was increased.