Complement activation was examined prospectively in 100 cardiopulmonary bypass (CPB) patients. Plasma C3a desArg (C3a) increased (cannulation: 234 +/- 33 ng/mL; 20 minutes on CPB: 622 +/- 51; 2 hours after CPB: 1143 +/- 109, p less than 0.0001). C3a at 2 hours was higher in the 13 patients requiring mechanical ventilation for longer than 1 day (1023 +/- 274) than in the 67 without respiratory complication (568 +/- 45, p less than 0.004). Five more patients were studied for neutrophil activation to confirm that a biologic effect of complement activation occurs during CPB; in these five patients C3a increased to 317% of baseline after 10 minutes on CPB with a corresponding rise in neutrophil cell surface receptors for the complement opsonin C3b (as measured by indirect immunofluorescence) to 168% (p less than 0.05). Both increases were sustained at 30 minutes. Temperature, dilution, and heparin were studied as variables relevant to CPB. Exposure of normal neutrophils to C5a in vitro caused an increase in C3b receptors which was dependent on temperature (0 specific fluorescence at 0 C, 30 at 25 C, 180 at 30 C, and 275 at 37 C). Generation of C3a and C5a in normal serum by zymosan was also temperature-dependent (ng/mL C5a generated: 0.7 at 25 C, 200 at 30 C, and 897 at 37 C; ng/mL C3a generated: 546 at 25 C, 10,872 at 30 C, and 65,667 at 37 C). Serum dilution to 33% decreased ng/mL C5a generated in the same system from 200 to 76 with no effect on C3a. Addition of heparin to 20 U/mL decreased ng/mL C3a generated from 10,872 to 913 and C5a from 200 to 8. Thus, hypothermia, dilution, and heparin protect CPB patients from complement activation by reducing both generation of C3a/C5a and the subsequent cellular response of neutrophil activation.