In vivo and in vitro experiments were under-taken to study factors influencing 99mTc binding from an EHDP complex to bone. In vivo 99mTc (IV) complexes of EHDP, ATP, citrate and ascorbate were injected into rats. The effects of pretreatment of rats with EHDP on EHDP- and ATP-Tc (IV) injection were examined. Citrate and ascorbate gave no bone scintigram, ATP gave a bone scintigram inferior to EHDP-Tc (IV). EHDP pretreatment deteriorated EHDP- and ATP-Tc (IV) scintigrams. Micro-autoradiography showed that Tc-activity is only found in the bone matrix of 15-day-old rats and that EHDP-pretreatment diminishes this labelling. In vitro experiments showed that EHDP also diminishes labelling of hydroxyapatite by EHDP-Tc (IV), but that an excess of Ca can augment this labelling. It is suggested that Tc (IV) binds to bone because EHDP binds to bone, permitting released Tc (IV) to bind to bone separately.