[en] The proper medical control of plutonium requires periodic evaluation of the lung burden of chronically exposed process workers. Instruments developed for this purpose and currently in use cannot detect levels below about one-half to one maximum permissible lung burden. Confidence in such measurements is limited by inadequacies in present calibration techniques. Most laboratories use different phantoms and different calibration methods; results from measurements on the same individual vary widely from one laboratory to another. A programme has been organized to develop methods of intercalibrating laboratories engaged in the in-vivo assay of transuranic elements. The first phase of the programme has been completed. The response of instrumentation and detectors to point sources of activity was determined in a number of laboratories. Current efforts are being directed toward the design and construction of a phantom to be used specifically for the calibration of detectors for the in-vivo detection of X- and low-energy gamma-ray emitters in the human lung. (author)