Complement and fibrinolytic factors were measured in 9 infants undergoing hemofiltration immediately after cardiopulmonary bypass in an attempt to reduce activation of these systems. Plasma levels of C3a, C5a, and terminal complement complexes increased during bypass by 460%, 85%, and 745%. Plasma levels were reduced after hemofiltration in 8 of the 9 infants, and C3a and C5a fractions were recovered in the ultrafiltrate. The observed activation of the fibrinolytic system seemed to be unaltered by hemofiltration. Fibrinolytic factors were not filtered. Our study shows that increased concentrations of complement factors in the plasma after bypass in infants may be reduced by hemofiltration.