Iodine-125-labeled antipyrine (¹²⁵I-Ap) (4-iodo-1,5-dimethyl-2-phenyl-3-pyrazolone) has been used in many laboratories as a label for organ or total-body water and for the estimation of organ blood flow using uptake or washout techniques. In our own use (1) we thought it imperative that the isotopic radical be firmly attached to the parent compound to prevent erroneous interpretation of the results.

Munck and Andersen (2) observed that up to 26% of the ¹²⁵I label became detached from ¹²⁵I-AP after dilution in saline. Previously we had not been concerned that this might be a significant problem because ¹²⁵I-Ap washout curves from isolated heart preparations showed no evidence of a diffusion limitation to washout, all of the curves being super-imposable when plotted on an abscissa of the product of actual flow and time (3). Since tissue permeability to halides is high compared, for example, with that of sodium and potassium, those experiments did not provide a critical test for the presence of ¹²⁵I⁻; and because we wished to use ¹²⁵I-Ap in future experiments and to compare its movement with that of labeled water, we needed to establish that the unbound fraction was small. Also we needed to choose a method that would be routine for us to use as well as one in which we could have confidence.