Abstract The bisphosphonate drug APD (pamidronate, 3-amino-1-hydroxypropylidene-1,1-bisphosphonate) has been shown to bind to human plasma proteins. This was an unexpected observation since this hydrophilic, anionic drug is not typical of molecules that exhibit this characteristic. At a concentration of 5 μg/ml the extent of binding of APD to fresh human plasma in vitro was variable between subjects 30.2% ± 8.5% (mean ± S.D., n = 10). Binding was not influenced by the time or concentration of APD over the range 0.05–10.0 μg/ml. At 20 and 50 μg/ml some precipitation of APD occurred. Both calcium and iron play a role in the binding of APD to plasma proteins, addition of calcium to plasma increased the degree of binding of APD, whereas the calcium chelators EDTA and EGTA reduced the binding of APD. Similarly, addition of iron to plasma increased the binding and the inclusion of the iron chelator desferrioxamine diminished the binding of the drug. The effects of iron and desferrioxamine were less pronounced than those of calcium and EDTA, indicating that the majority of the binding involves calcium ions and a smaller contribution is made by ferric ions. The equilibrium dissociation constants ( K d) for APD binding to calcium and iron binding sites on plasma proteins were estimated to be 852 μM and 29 μM, respectively. Calcium binding sites were of high capacity but low affinity and the iron binding sites were of lower capacity and higher affinity. Electrophoresis of plasma proteins following incubation with [ 14C]APD revealed binding to the transferrin and globulin fractions. However, there was some dissociation of protein bound APD during the electrophoresis. The consequences of hypercalcaemia on the pharmacokinetics of APD are discussed.