The possible occurrence of DIC in active systemic anaphylaxis was investigated in mice. Induction of active systemic anaphylaxis resulted in the development of DIC symptoms such as thrombocytopenia, prolongation of prothrombin time, hypofibrinogaemia, and elevated level of fibrinogen/fibrin degradation products. In addition, in histological examinations, massive congestion and cellular infiltration in pulmonary interstitia, and considerable haemorrhage in renal medullae were observed. All these changes were nearly completely prevented by pretreatment with platelet-activating factor (PAF) antagonist (BN 50739). Moreover, the same haematological and morphological changes were produced by a bolus injection of PAF. These data strongly suggest that DIC occurs in active systemic anaphylaxis and PAF plays a pivotal role in the development of DIC in anaphylaxis.