Symptoms, therapy, and outcome of 49 cases of anaphylactic reaction are presented. Epinephrine (0.5-2.0 mg) did not produce any circulatory improvement but induced severe dysrhythmias and even ventricular fibrillation. Neither epinephrine nor prednisolone (2 g) prevented shock development, neurologic sequelae, or fatal outcome. Rapid infusion of 2000 ml 50% colloid (dextran 70 or starch, m.w. 450,000) with 50% Ringer's lactate, however, reliably restored circulation within 30 min. If elevated, airway resistance dropped in conjunction with colloid administration rather than with any other drug. Impairment of the pulmonary circulation is found to be the initial feature of anaphylaxis, manifested by a rise in pulmonary vascular resistance; then stroke volume and systemic blood pressure fall as left heart filling is reduced. Many findings indicate a high blood viscosity with resulting capillary occlusion: rapid clotting; low levels of factors I, II, V, VIII, and X; low plasma protein, which may be only 50% of its normal value; thrombocytopenia; and aggregation of white blood cells with fibrin in small lung vessels. Blood and plasma vanish from the circulation and appear to be sequestered in the pulmonary capillaries as the initial response to the antigen; later, the whole body is affected. Congestion and pain in the lymph nodes indicate that the lymphatic system may also become involved. On the basis of hemoconcentration, a blood volume deficiency in the range of 30% has been calculated.(ABSTRACT TRUNCATED AT 250 WORDS)