In 16 patients we have carried out simultaneous plasma-volume measurements with human serum albumins tagged with Tc-99m (Tc-99m HSA) and I-131 (I-131 HSA). The correlation coefficient was 0.987. Tc-99m HSA, prepared from kits that predictably yield high labeling efficiency (and thereby negligible amounts of TCO4-), is clearly a superior agent for repeat plasma volume determinations, because of its shorter half-life and the reduced radiation dose to the subject.