Abstract In 1915, volunteers were injected with, or ingested, solutions of a radium salt, and the excreta were analysed for radium, but the first direct demonstrations of the presence of radium in vivo years after exposure were reported in 1925 by Martland et al.; they were also able to demonstrate the presence of radon isotopes in the exhaled breath. In 1929, Schlundt et al. reported radium levels in six living persons based on a calibration that was derived from measurements on a seventh who subsequently died and whose partial skeleton was analysed. In 1933, Ives et al. reported results for 40 subjects who showed gamma-ray results of 1 μg radium (i.e., retained radon) or more. There was an improvement in sensitivity by an order of magnitude in 1937 when Evans described the use of a Geiger-Müller detector and the metre-arc geometry; the method was still incapable of measuring radium at the “tolerance dose” of 0.1 μg. Ten years later, Hess and McNiff published details of an ionization chamber method that did permit such levels to be determined. In the early 1950s, three groups independently described the usse of long high-pressure ionization chambers that were sensitive enough to detect the radiation from the naturally radioactive potassium content of the human body. In 1955, the introduction of sodium iodide as gamma-ray detector, with its high detection efficiency and spectrometric capabilities, and of large organic scintillators, revolutionized the assay of radium (and all other gamma-ray emitters) in vivo. The lower limit of detection of radium in man showed a steady decrease that was roughly exponential from 1929 to the mid-1950s with a half-period of a little more than 3 y.