123I-Granuloszint (a murine monoclonal antibody--called AK-47--against NCA-95 glycoprotein of granulocytes) has been proved to be a very convenient and successful radiopharmaceutical for visualizing infectious diseases. For a broad introduction in routine nuclear medicine it was necessary to optimize the labelling method and to determine in vitro exactly those biological and binding parameters which are relevant for an effective application in vivo. Binding to granulocytes has been shown to be specific and saturable (non specific binding about 10%) and is not via the Fc part of the antibody. The investigation of the binding properties of 125I-labelled AK-47 gave the following results: affinity constant 5 x 10(8) M-1, 20,000-200,000 epitopes per granulocyte and an immunoreactivity of more than 90%. Labelling with 123I reduced the immunoreactivity to 40%. The Lindmo method and immunoblotting are used as quality control to check the likely in vivo behaviour of the labelled antibody. There is a good correspondence between the results from the two methods. With our special labelling method and the different in vitro tests we have found a reliable way to control the production and to assure an optimal binding behaviour of 123I-Granuloszint.