The sole purpose of radio-contrast agents is to increase the absorption of X rays in blood vessels and soft tissues. Current diagnostic agents take advantage of the properties of iodine with its acceptably low clinical toxicity and moderately high atomic number (53), the latter yielding a relatively large absorption coefficient for photons in the diagnostic X-ray energy range, owing to the photoelectric effect. One consequence of this, not often appreciated, is that cells in the vicinity of an iodine-rich solution will receive a higher radiation dose than would be the case if iodine were not present. This is partly responsible for the reported cytogenetic changes in lymphocytes in vitro and in vivo (Adams et al, 1977; Norman et al, 1978; Cochran et al, 1980; Cochran & Norman, 1982; Hadaghy et al, 1982). Though these data clearly signal an undesirable, if unavoidable, phenomenon in diagnostic radiology, such a cytotoxic effect may be capable of exploitation in radiotherapy. To examine this latter possibili...