The origins and current activities of the United States Transuranium and Uranium Registries are traced with emphasis on biokinetic modelling, and new biokinetic models, for plutonium and americium, are proposed suitable for operational applications. Although started more than 25 years ago, the Registries remain the only formally organised human tissue research programme devoted to the study of actinides in man. Using radiochemical techniques, the Registries determine the amount of actinide elements in the tissues of volunteer donors at autopsy, using this information along with the individual's exposure and medical history, to evaluate the distribution, translocations, excretion and dose from Th, U, Pu, and other actinides in the body. The results of the Registries studies indicate that in addition to skeleton and liver, muscle constitutes and appreciable reservoir for both Pu and Am, although the dose to muscle is relatively low because of its large mass. The systemic biokinetics of Pu and Am are well represented by a three-compartment model representing skeleton, liver and other soft tissue (primarily muscle) with different biokinetic constants for the two nuclides. For Pu, the initial uptake fractions are 0.4, 0.4, and 0.2 with residence half-times of 50, 20 and 10 years, respectively for the skeleton, liver, and muscle. For Am, the comparable uptake fractions are 0.45, 0.25, and 0.3 with residence half-times of 50, 2.5 and 10 years for the three compartments. Further to encourage radiobiology studies as well as to provide unique materials for research in other areas such as biomarkers and oncogenes, the Registries have established the National Human Radiobiology Tissue Repository, which includes more than 20,000 tissue samples plus a group of tissue solutions from the general American population. On request, these materials are available to other investigators for collaborative or individual research purposes.