This paper presents evidence that the fraction of radioactive iodide (*I-) present in a sample of radioiodinated protein is grossly underestimated if determined by trichloroacetic acid precipitation in the absence of excess unlabeled iodide ion. In addition, the widely used separation of *I- from radioactive iodine-labeled iodotyrosine in supernatants of trichloroacetic acid precipitates, which proceeds by oxidation of *I- and subsequent extraction into chloroform (24), is dependent on the concentration of trichloroacetic acid used. Reproducibility and accuracy appear to be obtainable at concentrations of 10% (w/v) or higher. The modification of Goldberg's thin-layer chromatographic method (25) presented as an alternative method in the current paper offers the simplicity of separating *I-, *I-Tyr, and radioiodinated protein in a single step. The method can be used for small samples of urine, plasma, or tissue homogenates in which quantification of the labeled by-products from radioiodinated proteins is sought.