The discovery of a number of phosphate complexes labelled with 99-Tc-m that localize in bone has aroused wide-spread interest in bone scanning. The physiological properties of these and other clinically useful bone-seeking radiopharmaceuticals are compared, and their physical properties assessed in relation to the characteristics and limitations of avilable detector systems. A hypothesis is put forward to explain the behaviour of the technetium-labelled agents. It is concluded that although there are differences in biochemical behaviour between these agents, strontium and fluorine, all three may, under suitable conditions, give similar clinical information. The radiation dose received by the patients is least with the usual dose of 99-Tc-m and the blood clearance of the diphosphonate and pyrophosphate preparations is faster than that of strontium, although slower than fluorine. The psi-ray energy of technetium permits a much greater efficiency of detection than of fluorine. These factors, toghether with the general availability of 99-Tc-m and its relatively low cost make the technetium diphosphonate or pyrophosphate preparations the agents of choice for most skeletal radioisotope imaging. However, there are as yet insufficient follow-up studies to be able to assess the incidence of either false-negative or false-positive findings with these agents.