A nanocolloid marked with 99mTc was used to perform 100 bone marrow scintiscans in 72 patients given radiotherapy for tumours. The compound is taken up by the cells of the phagomonocytic system whose distribution in the bones is similar to that of the erythropoietic marrow. With doses over 30 Gy a more or less marked reduction in captation by the marrow involved in radiation was always noted. The damage remains even for years after radiation, though recovery is possible especially in young patients given moderate doses of up to 40-45 Gy. Bone marrow scintigraphy using a nanocolloid is noninvasive and inexpensive, involves only tiny radiation doses and appears to be useful in the study of cancer patients subjected to radiation treatment either alone or in combination with chemotherapy.