The dose received by the active bone marrow during certain diagnostic procedures is of considerable interest in this present era of ever increasing exposure of the general population to ionising radiation. The dose to the hemopoietic organs has received considerable attention. Previous measurements of dose to bone marrow have been made with small-volume condenser chambers especially developed to have a uniform energy response in the region of interest (1, 2). The disadvantages of using an ionisation chamber for measuring energy absorbed in biological sites are: (a) disturbance of the biological medium by the introduction of the air equivalent and air-filled chamber, (b) the difficulty of achieving satisfactory energy response to take account of the low-energy degraded radiation at the site, and (c) the considerable polar dependence of most ionisation chambers. This work describes the measurement of absorbed dose in the 7th and 8th thoracic vertebrae in a human phantom at x-ray energies, using a sensitive ...