Complement conversions and a pulmonary leukocyte sequestration were observed during cardiopulmonary bypass (CPB) in all of twenty patients who received open heart surgery. A systemic neutropenia was initially observed in the early phase of CPB, and it subsequently turned into a systemic neutrocytosis during the late phase of it. A significant leukocyte sequestration was found in the pulmonary circulation during CPB coincidently with the systemic neutropenia. The contribution of the transpulmonary sequestration to the leukopenia was major, being as high as 80 per cent. Plasma C3, C4, and CH50 levels rapidly decreased following the commencement of CPB. Alterations of the C3 molecule in the patients' plasma were demonstrated using the immunoblotting method. The appearances of C3a and C3b with the apparent molecular weights of 10,000 and 170,000 respectively, might be evidence of the complement activation during CPB. Another type of alteration in the C3 molecule was observed in the generation of a new fragment with the apparent molecular weight of 14,000. The appearance of this fragment, which did not share common epitopes with C3a, might suggest the consumption of complement components during CPB, unrelated to the activation of the complement systems.