A new approach to radiopharmaceutical design is demonstrated, in which small chelating groups capable of binding .gamma.-emitting radiometals are attached to biologically active molecules, thus producing radiopharmaceuticals based on bifunctional drug and biochemical analogs. The chelating group iminodiacetic acid was evaluated for this role by examining 2 N-substituted iminodiacetic acids: methyliminodiacetic acid (MIDA) and N-(2,6-dimethylphenylcarbamoylmethyl)iminodiacetic acid (HIDA). Radiochemical and biologic studies showed that both agents were obtained in high radiochemical purity, were stable in vitro and in vivo, and possessed biologic distributions governed almost exclusively by the N-substituted group. These characteristics of 99mTc-labeled N-substituted iminodiacetic acids, prepared using an instant kit method, provide the basis for a valuable new class of radiopharmaceuticals based on bifunctional drug and biochemical analogs.