Abstract Quinuclidinyl benzilate, a muscarinic antagonist, has previously been used in its tritiated form ([ 3H]-QNB) to study the lung muscarinic receptor. We investigated whether a newer iodinated form of QNB ([ 125I]-QNB) of higher specific activity would be an appropriate ligand to study the human peripheral lung muscarinic receptor. Both the tritiated and iodinated ligands bound specifically to human lung at 23°C. At 37°C the specific binding of [ 3H]-QNB increased slightly, but no specific binding of [ 125I]-QNB was found. The data from multiple equilibrium binding experiments covering a wide range of radiolabeled QNB concentrations were combined and analyzed using the computer modeling program, LIGAND. The tritiated QNB identified a single affinity human lung binding site with a Kd of 46 ± 9 pM and a receptor concentration of 34 ± 3 fmol/mg protein. The iodinated QNB identified a single higher affinity human lung binding site (Kd = 0.27 ± 0.32 pM) of much smaller quantity (0.62 ± 0.06 fmol/mg protein). Competition studies comparing the binding of unlabeled QNB relative to labeled QNB indicated that unlabeled QNB had the same Kd as that measured for [ 3H]-QNB, but a 5 log greater Kd than that measured for [ 125I]-QNB. Other muscarinic receptor agonists and antagonists competed with [ 3H]-QNB, but not [ 125I]-QNB for binding to muscarinic receptors with the expected magnitude and rank order of potency. We conclude that of the 2 radiolabeled forms of QNB available, only the tritiated form should be used to study the human peripheral lung muscarinic receptor.