Lately, the specific binding of appropriate monoclonal antibodies to human granulocytes has been used for the scintigraphic detection of inflammatory foci. Using the antibody BW 250/183 we studied the underlying binding kinetics. As an important requirement for a specific cell binding it has been shown that the labelling procedure does not change the immunoreactivity of the antibody. An affinity constant of 2 x 10(9) l/mol has been calculated from binding studies. Usually, 0.25-1.0 mg of the 99mTc-labelled antibody are applied per patient. In the present study an equilibrium in blood appeared quickly after intravenous application; at steady state about one fourth of the activity was cell-bound. The rest of the activity circulated in the plasma in the form of labelled IgG and was able to react directly with those granulocytes which were already accumulated in the inflamed area. Even a drastic reduction of the applied protein mass did not change this equilibrium. The law of mass action seems not to be directly applicable to this problem. Interferences with components of the plasma can be excluded as explanation for this behaviour. After application of in-vitro labelled granulocytes from which unbound antibodies were removed completely by washing, an identical steady state was observed within 10 min after injection; however, in this situation the intravasal residence time of activity increased distinctly.