The total haemolytic complement (CH50), the complement components C3 and C4, the complement breakdown product C3d, alternative pathway activation and transferrin, were measured before, during and after cardiopulmonary bypass. As expected, CH50 decreased after heparinization, remained low during bypass and decreased further up to 8 h after bypass. C3 and C4 decreased significantly during bypass, continued to decrease for a further 8 h after bypass (by 35% and 40% respectively) and thereafter increased gradually up to 48 h. Although the depletions observed were suggestive of complement activation, there were no demonstrable increases in C3d, and in all patients the concentration of C3d remained within the normal range. Hence it was concluded that complement depletions of this magnitude were unlikely to result from complement activation. Non-specific changes in protein concentrations during bypass, as a result of dilution, redistribution or other unidentified factors, are more probable causes of the observed reductions. The acute phase response to surgery may be a factor in the subsequent increase in C3 and C4 which is seen 24 h after bypass. As transferrin concentrations in the plasma are known to decrease during this response the observed decrease in transferrin concentration would support this view.