Peripheral mononuclear leukocytes (pMNL) bear a population of beta 2-adrenoceptors. Radioreceptor assays with (-)-125Iodocyanopindolol (125I-CYP) are often used to determine the expression of these hormone receptors under physiological and pathological conditions. Doubts on the occurrence of just one class of binding sites as well as the availability of new laboratory equipment prompted us to revise the procedure employed for investigation of these receptors. pMNL were harvested from venous human blood by density centrifugation with LymphopaqueR, LymphoprepR, or FicollR yielding immunologically distinct pMNL fractions. Receptor binding assays were performed semi-automatically with 125I-CYP in the range from 0.6-600 pmol/l. Analysis of the data (modified affinity spectra, Scatchard plot) revealed two classes of binding sites (high- and low affinity binding). The binding isotherms were sigmoidal in the concentration range from 0.6-3.0 pmol/l. Parameters estimated for the high affinity binding site may vary by a factor of 10, depending on the mathematical model employed.