Monolayers of Chinese hamster lung cells (CCL-16) in a polystyrene phantom were irradiated in vitro by 103Pd and 125I sources at dose rates of 6 to 72 cGy/h. Cell survival curves for acute high-dose-rate irradiation (over 30 Gy/h) were also measured using nearly monoenergetic X-ray beams which were designed to simulate the mean energies of photons emitted by 125I and 103Pd and also using a clinical 250 kVp X-ray beam. A profound dose-rate effect is observed over the dose-rate range of 6 to 20 cGy/h. An inverse dose-rate effect was observed for both radionuclides, with its onset occurring at a dose rate of about 20-30 cGy/h. The average RBE of 103Pd relative to 125I was determined to be 1.45 +/- 0.07, 1.41 +/- 0.07, 0.70 +/- 0.07 and 1.49 +/- 0.07 at dose rates of 6.9, 12.6, 19.0 and 26.7 cGy/h, respectively. Because 103Pd implants are generally prescribed at a higher initial dose rate (21 cGy/h) than the corresponding 125I implants (7 cGy/h), the effects of both dose rate and photon energy on biological response must be considered together. For the CCL-16 cells, the RBE of 103Pd at 19.0 cGy/h relative to that of 125I at 6.9 cGy/h was estimated to be 2.3 +/- 0.5.