On the basis of dose readings in 102 patients treated with total body irradiation (TBI), a 'tissue air ratio (TAR) curve' has been produced. It could be useful to precalculate treatment time in TBI, for dose prescription to a specific point, provided the same source (60Co) and treatment setting (lateral irradiation; 3 m source-axis distance; reference point at thighs bifurcation, near the perineum) is used. The TAR curve produced, and the formula relating tissue depth to TAR value, are presented, and compared to preexisting data for 'magna fields' treatments. This curve is exponential, and in semilog representation becomes straight, as every classic TAR curve; it is lower than others, reflecting non full-scatter situation in patient irradiation.