The process o f ageing is associated with increased susceptibility t o infection. Phagocytes form the primary defence mechanism against infecting microorganisms, but the influence o f ageing on phagocyte function remains controversial. In this study w e have applied a microtitre plate phagocyte chemiluminescence (CL) assay suitable for clinical use t o compare phagocyte oxidative metabolism in younger healthy subjects (age 20-60 years) and healthy older (60-70 years) subjects. Polymorphonuclear leukocytes (PMNL) and monocytes were stimulated using phorbol myristate acetate (PMA), serum opsonized zymosan (SOZ), and non-opsonized zymosan (ZYM) in the presence of both lucigenin and luminol. Monocytes showed a higher luminolenhanced CL response t o PMA in males compared with females in the younger age group. No PMNL differences were observed between the sexes. Although no differences were found in relation t o age when cells were stimulated w i t h PMA and SOZ, significantly lower background (unstimulated) CL was obtained from PMNL with luminol. PMNL luminol-enhanced CL responses were also lower in response t o ZYM. The findings suggest a reduced response o f PMNL from older subjects t o minimal stimulation. This could be related t o abnormalities i n the triggering o f the respiratory burst or myeloperoxidase release due t o ageing. The influence o f age and sex should be taken into account in clinical studies o f phagocyte CL.