We have developed a partially automated method for the performance of equilibrium radioligand binding studies which is applied by our group in investigations on circadian variations and stimulation studies on beta 2-adrenoceptor sites in human peripheral mononuclear leucocytes (pMNL). Using a Tecan Robotic Sample Processor, binding assays with 12 concentrations of 125iodocyanopindolol (1-150 pmol/l, total binding in triplicates, unspecific binding in the presence of 10(-5) mol/l timolol in duplicates) are prepared automatically with all titer tubes per experiment arranged in a microtiterplate-sized rack. After incubation in a waterbath for 2hr at 37 degrees C, the whole rack is centrifuged at 5000g and transferred back to the lab robot. Bound radioactivity is separated from the unbound ligand by removing the supernatant by the machine. The radioactive counts are evaluated using personal computers. The lab robot enhances reproducibility of experimental results and frees lab workers from time-consuming pipetting jobs. Radioactive exposure is minimized to the time preparing the radioligand working solution and transferring the sample tubes from the robot to the waterbath, to the centrifuge and back to the robot. The variability of our software allows easy adaptation to other binding studies with intact cells.