A computer program is presented for non-linear regression analysis of radioimmunoassay data, which allows the determination of the binding parameters of monoclonal antibodies, i.e., affinity constant and number of available binding sites. The program is based on the median method and gives the values of the binding constants and their 95% confidence limits, without the necessity for their preliminary, approximate evaluation. The program may also be used to determine the binding constants for the interaction of receptors with labelled ligands.