The authors evaluated the stress response to cardiopulmonary bypass by measuring plasma Ig G, A, M and C3, C4concentrations. Anesthesia was induced with fentanyl, and all patients were underwent open heart surgery using bubble type heart-lung machine. Blood samples were obtained pre-induction, sternotomy, 10 min. and 30 min. after cardiopulmonary bypass, and 10 min. after termina tion of cardiopulmonary bypass and after transport to L.C.U. IgG, A, M and complement C3, C4concentrations were measured by Turbidmeter using Behring Turbidquant. The results were as follows: 1) Plasma concentrations of IgG, A, M and C3, C4 at preinduction period were 1345. 00±194. 42, 289. 10±100.4, 177.43±72.65, 59.20±16.33, 20.50±7.54 mg/dl., respectively. 2) Ten min. after cardiopulmonary bypass, the plasma concentration was significantly decreased when compared with the baseline values (p<0.01). 3) Thirty min. after cardiopulmonary bypass, the plasma concentration of the complenent C3, C4 were significantly decreascd (p<0.01). 4) In the 1ntensive care unit, Ig and complement C3, C4 were increased, but did not reach control values. Considering the above results, we conclude that the decrease of plasma Ig G, A, M and complenent C3, C4concentrations are related to mechanical stress of the cardiopulmonary bypass, and to certain activators such as denaturated protein, heterogenous RBCs, and foreign materials, etc.