Two 99mTc-labelled radio-diagnostics for bone scintigraphy, dicarboxypropan-diphosphonic acid (DPD) and methylene-diphosphonate (MDP) were compared. The test parameters were the time-activity curves of serum and of deproteinised serum, time-activity curves in regions of interest above the femur, the sacrum and the soft tissue medial of the femur, and the urinary excretion. The ratio of bone lesion to normal bone was compared for 6 h after the injection of each compound. The time-activity curves in the serum and the deproteinised serum were not very different, therefore the percentage of the radio-diagnostics bound to protein in blood were nearly the same. At 14 h post-injection nearly 80% of the activity remaining in the blood pool was bound to protein. The urinary excretion of MDP was 25% higher than that of DPD, because the renal clearance of DPD was 41% lower than that of MDP. The non-excreted activity was bound to bone; therefore in the regions of interest set above the sacrum, the femur and the soft tissue the activity ratio was 27% higher for DPD for the ratio os sacrum to soft tissue and 21% higher for the ratio femur to soft tissue, but there were similar results for both substances for the ratio bone lesion to normal bone; therefore MDP was not better than DPD in detecting bone lesions.