Tracer amounts of [59Fe++]citrate, [111In+++]chloride, and [68Ga+++]chloride were complexed with autologous plasma transferrin. Each of these complexes were co-administered with [125I]albumin by i.v. injection and their biodistribution was studied in Wistar rats. The plasma clearance of 59Fe and [125I]albumin was monoexponential with half-times of 49-70 and 277 min, respectively. The plasma clearance of 68Ga and 111In was biexponential with second component half-times of 157 and 232 min, respectively. Indium-111 tissue distribution was similar to that of [125I]albumin in heart, lung, muscle, brain and Walker-256 allograft. Iron-59 distribution spaces were generally the highest of the metal complexes in all tissues except muscle, where the 68Ga space was highest. The effects of transferrin-specific receptor-mediated endocytosis can be avoided in many organs and Walker-256 allografts by using the indium-transferrin complex, and the radiolabeled complex may be a convenient macromolecular tracer to estimate vascular permeability and vessel pore size in tumor and systemic tissue. In contrast, the iron-transferrin complex may be useful for measuring and imaging transferrin-specific receptors in brain and tumor tissue.