The potential mechanism of fluorocarbon emulsion (FCE)-induced anaphylactoid reactions characterized by transient flush and respiratory disturbance in a few patients were elucidated by detecting the immunological response and the blood vessel vasoactive amine content. The passive cutaneous anaphylaxis (PCA) test in rats immunized with FCE was negative. The precipitating FCE reaction antibody (FRA) in sera of rabbits at 20 min and 24 h after i.v. infusion of FCE 10 ml/kg by reversible single radioactive immunodiffusion (RSRI) was positive, the densities of FRA were negatively related to the dilution of FCE (24 h, r = -0.9998). Histamine and 5-hydroxytryptamine (5-HT) content in peripheral blood of rabbits using fluorochromatography were decreased by 79% and 92% respectively at 10 min after i.v. infusion of FCE 10 ml/kg. Platelet counts fell to 67%, WBC to 47% and the ratio of polymorphonuclear cells and lymphocytes (P/L) was depressed by as much as half of that before infusion. Small arterioles of lungs were filled with macrophages. These results indicated that the mechanisms of FCE-induced anaphylactoid reactions did not involve IgE-dependent mediator release (histamine and 5-HT), but was associated with FCE-FRA complex formation and macrophage-stasis in pulmonary vessels.