With the use of two mammalian cell lines (L5178Y and V79), a comparison was made of the efficiency of cell killing by gamma rays and beta particles from tritiated water and incorporated tritiated thymidine. For the latter, cells were incubated with tritiated thymidine for one generation (unifilar labeling) or for four generations (bifilar labeling). To prevent cell division during exposures, irradiations were carried out with cells held in the frozen state or, in one case, at 5°C. There was no significant difference in cell killing efficiency for unifilar or bifilar modes of tritiated thymidine incorporation. In the frozen state an increase in D0 was observed for all modes of irradiation, but an apparent increase in killing efficiency was observed for irradiations following incorporation of tritiated thymidine relative to that observed for irradiations with gamma rays. For cells irradiated at 5°C, the efficiency of cell killing by beta particles from tritiated water or incorporated tritiated thymidine was not appreciably different, but both were more efficient than gamma irradiation delivered under similar conditions. For killing of V79 cells at 5°C, and at a dose rate of 20 rad/hr, the relative biological effectiveness (RBE) of tritium beta particles relative to cobalt-60 gamma rays was estimated to be approximately 1.7-1.9.