When utilizing iodohydroxybenzylpindolol (IHYP) as an adrenergic receptor probe in muscle membrane systems, the data demonstrated an unacceptably high nonspecific binding component. Bearer et al. have reported that chloramine-T induced iodination of hydroxybenzylpindolol (HYP) results in the incorporation of iodine into the indole ring rather than into the phenolic moiety as noted previously by others. These results suggest that pindolol itself can also be iodinated. Therefore, the usefulness of carrier free 125I-labeled iodopindolol (IPIN) as an adrenergic receptor probe was investigated. Using between 0.01 nM and 0.1 nM [125I]IPIN in two different muscle membrane systems, we found the nonspecific binding component to be 10% or less of total binding. When [125I]IPIN was used with membranes prepared from rat skeletal muscle, we found it to interact with a single set of high affinity binding sites (KD = 0.13 +/- 0.01 nM) with the characteristics of beta adrenergic receptors and a density of 48.5 fmoles/mg protein. IPIN binding was also studied with purified dog cardiac sarcolemma. A single set of binding sites was detected having a KD of 1.64 +/- 0.5 nM; the density of these sites was 289 fmoles/mg membrane protein. [125I]IPIN may be a useful probe for the beta adrenergic receptor of tissues in which [125I]IHYP and other beta adrenergic receptor probes have a non-specific binding component which approaches that of the specific binding component.