1. Oxidative damage has been associated with ageing, but there is no agreement as to whether or not it is produced by a decrease in antioxidant defences with the ageing process. In purified lymphocytes from 47 healthy elderly (75.27 +/- 0.91 years) and 47 healthy young (29.87 +/- 0.53 years) volunteers, we studied the levels of antioxidant enzyme activity (superoxide dismutase, catalase and glutathione peroxidase), protein oxidative damage (as protein carbonyl content) and lysosomal proteolytic activity (cathepsins B, H and L), with and without exposure to oxidative stress produced by 25 mumol/l H2O2. 2. There were no differences in antioxidant enzyme activities in the stressed and non-stressed samples between the young and elderly subjects, indicating that there was no relationship between age and antioxidant enzyme activity even in oxidative stress. However, a dissimilar response to oxidative stress was observed in protein oxidative damage and cathepsin B and L activities, depending on the age of the donor. 3. With these results we conclude that oxidative stress produces greater protein oxidative damage and increased protein degradation in elderly subjects than in young ones; this effect cannot be attributed to dissimilar antioxidant enzyme responses to oxidative stress, since these did not differ between the two age groups.