The antioxidant capacity of plasma taken from 10 patients before and during propofol anaesthesia was measured. Mean total plasma antioxidant activity fell from 1.73 to 1.64 mM L−1 trolox equivalents (P=0.047). This was caused by haemodilution since mean haemoglobin concentration fell from 13.0 to 12.5 g dL−1 (P=0.016). The time to 50% haemolysis (H50) of 10% red blood cell suspensions induced by 2,2'azo-bis(2-am-idinopropane) dihydrochloride (ABAP) was measured in blood from six patients. There was no change when red cells alone were studied, but when the patients' plasma was added to red cell suspensions (producing 10% plasma and 10% red cell suspensions), mean H50 increased from 291 to 308 min (P=0.049). Despite there being no overall increase in plasma antioxidant activity, the lipid soluble component of blood antioxidant activity appears to be increased by propofol.