We examined the effect of aprotinin on the production of interleukin-8 and the expression of leukocyte adhesion molecules in an isolated cardiopulmonary bypass circuit. Five healthy volunteers each donated 500 ml blood which was divided into equal portions (A) and (B). Portion A was treated with aprotinin 250 000 kallikrein inactivator units and added to a circuit primed with a further 250 000 kallikrein inactivator units. Portion B was simultaneously treated with 25 ml normal saline and added to an aprotinin-free circuit. Samples of blood were taken at donation, after addition of a drug and at 30-min intervals throughout isolated cardiopulmonary bypass. The interleukin-8 increase was smaller in the aprotinin group but the difference between groups was not statistically significant (p > 0.05). In contrast, aprotinin inhibited the cardiopulmonary bypass-induced increase in monocyte (p < 0.05) and granulocyte (p < 0.01) CD18 and the increase in granulocyte, but not monocyte, CD11b (p < 0.05). Cardiopulmonary bypass had little effect on expression of L-selectin. These results confirm that aprotinin attenuates leukocyte activation during isolated cardiopulmonary bypass.