In 119 children, predominantly newborns and babies with sepsis, alpha 2-Antiplasmin was determined by the use of the chromogenic substrate S-2251. In healthy newborns, the inhibitor level averaged 65 per cent of the adult level. Already in the initial phase of sepsis, enhanced alpha 2-antiplasmin values were observed. During the further course, they increased markedly. Thus, alpha 2-antiplasmin proved to be an acute phase reactant together with fibrinogen, factors II and X, and alpha 1-antitrypsin measured as trypsin inhibitor capacity. The correlation analysis in all subgroups showed moderately tight binding to fibrin. In patients with shock or in those who decreased, lower levels were measured. The overproduction is assumed to be caused by disseminated intravascular coagulation processes. In other diseases such as respiratory distress, alpha 2-antiplasmin was reduced. In case of disseminated intravascular coagulation that was not caused by sepsis consumption of components dominated. In the probability paper, distribution of the values of the subgroups was found to differ markedly. Thus, the inhibitor proved to be of diagnostic value.