Haematological and histological studies were made following challenge in 8 monkeys (Macaca irus) sensitized with ovalbumin. Haemagglutinating, but no reaginic antibodies to ovalbumin were demonstrated before challenge. Both platelet and leucocyte counts decreased markedly (-80 and -89%, respectively) within 5 min after challenge. No evidence of haemoconcentration was found. Plasma fibrinogen decreased (-74%), indicating activation of the coagulation system. Histological examination revealed platelets, leucocytes, fibrin and a large number of hyaline globuli ('globular microemboli') in pulmonary capillaries and small arterioles. From their staining properties the globuli were considered to consist mainly of fibrin. Some globuli were also found in the liver and kidneys, but were rarely seen in other organs. It is concluded that the anaphylactic reaction in this experimental model can be classified as aggregate anaphylaxis, and it is suggested that platelet mediators play an important role in the early pulmonary vascular and respiratory response.