Apparatus and dosimetry techniques have been developed which make possible studies of the biological effects of radiation from encapsulated 125I sources at clinically relevant dose rates using mammalian cells attached to culture dishes. The variation of dose rate from 125I photons as a function of distance from the interface between different materials was investigated. A polystyrene substrate changes the mean dose rate in attached cells by about 21%, depending on cell thickness. To reduce dosimetry uncertainty caused by this effect, special petri dishes were made from polyvinylidene fluoride, which changes the mean dose in attached cells by only 10%. Chinese hamster ovary cells attached to these dishes were cultured in incubators which contained 125I and 137Cs sources, allowing the effects of various dose rates (.005 to 0.80 Gy/hr) of radiation from the two isotopes to be compared. The relative dose rates from these low- and high-energy photons were measured with an accuracy of +/- 7% or better using an air-equivalent ionization chamber designed to resemble one of our special petri dishes. Calculations of dose rates from 125I give values within 4% of the measured dose rates used to determine the relative biological effectiveness of 125I photons.