Kinetics of neutrophil inactivation was investigated in vitro by Nitroblue tetrazolium (NBT) test in the process of their contact with the substrate. It has been shown that the previous thermostatation results in an exclusive inactivation of neutrophils with high reaction ability leading to their complete inactivation. Such an inactivation is a consequence of cell contacts with the substrate, whose chemical structure and physicochemical properties define the process regularities. The neutrophil inactivation is probably not a consequence of the contact itself but may follow the next scheme: stimulus (contact with substrate)--generation of reactive oxygen metabolites--inactivation. Two functional unequal classes of neutrophils were differentiated on the basis of different levels of their reactive oxygen metabolite generation, and on their ability to inactivation. In vitro cells of one of these classes actively generate reactive oxygen metabolites to be inactivated in consequence of interaction with the substrate, whereas cells of the other class produce reactive oxygen metabolites less actively and are nor inactivated. Evidently, in vivo cells of the are phagocytes and those of the latter fulfill other functions.