Summary(1) Growth curves were obtained for a tetraploid strain of the Ehrlich ascites tumour of the mouse, both untreated and after irradiation.(2) Higher total cell-counts were obtained when viable and heavily-irradiated cells were mixed. This can be explained by the multiplication of the heavily-irradiated cells (which was demonstrated), rather than by stimulation of the growth of the viable fraction.(3) The method as a means of assessing cell viability after irradiation is compared with a cell-titration technique.