L1210 leukaemia cells were labelled with various doses of 67Ga-citrate, 3H-thymidine, or 125I-iododeoxyuridine to evaluate the cytocidal effects of the intracellular decay of the three radionuclides. Based on radioisotope incorporation data, cellular dimensions, and intracellular radioisotope distributions (3H and 125I intranuclear, 67Ga cytoplasmic) the rates of deposition of cellular, nuclear, and cytoplasmic energy were calculated. In terms of energy absorption/cell, 67Ga (LD50: 2250 keV/hr; 69 rad/hr) was much less toxic than either 3H (LD50: 325 keV/hr; 10 rad/hr) of 125I (LD50:50 keV/hr; 1-5 rad/hr). In terms of energy absorption/nuclesu, 67Ga and 3H produced almost identical effects (LD50: 230 versus 255 keV/hr; 22-2 versus 24-6 rad/hr), but 125I remained much more toxic (LD50: 40 keV/hr; 3-9 rad/hr). These findings indicate that, although decay by electron capture in the cell nucleus (125I) is highly destructive, the same type of decay occurring in the cytoplasm (67Ga) is ineffective in killing L1210 cells. An analysis of the data suggests that the cytotoxic effects of the three radioisotopes result exclusively from nuclear damage. Cytoplasmic absorption of radiation energy appears to contribute little, if anything, to the lethal effects of ionizing radiations.