The ability of dietary supplementation with carotenoids to protect chick tissues against oxidative stress in vitro was examined. Male Leghorn chicks were fed on diets supplemented (100 mg supplement/kg diet) with either β-carotene, zeaxanthin (β,β-carotene-3,3'-diol), canthaxanthin (β,β-carotene-4,4'-dione) or α-tocopherol, or on a control diet, from 1 d old until 37 d of age. Tissues (liver, heart, skeletal muscle and plasma) were removed and assayed for total carotenoids and α-tocopherol content and portions subjected to oxidative stress by incubation of homogenates with cumene hydroperoxide and FeSO4. Animals receiving zeaxanthin and canthaxanthin had significantly greater carotenoid concentrations in liver, heart, muscle and plasma compared with untreated controls (P < 0·05); animals fed on diets supplemented with β-carotene or α-tocopherol did not have significantly different tissue carotenoid contents compared with untreated controls. α-Tocopherol supplementation elevated α-tocopherol levels in all tissues examined (P < 0·05). Supplementation with caroteuoids did not affect tissue α-tocopherol levels, but β-carotene lowered plasma α-tocopherol levels by 50% (P < 0·05). Incubation of plasma or tissue homogenates with oxidant stressors induced lipid peroxidation (production of thiobarbituric-acid reactive substances) in all tissues. Animals given α-tocopherol, β-carotene or zeaxanthin had a reduced susceptibility to oxidant stress in liver compared with unsupplemented controls (P < 0·05), and α-tocopherol-supplemented animals had reduced susceptibility in skeletal muscle compared with unsupplemented controls (P < 0·05). Canthaxanthin supplementation did not influence the susceptibility to oxidant stress in any tissue examined. These results suggest that zeaxanthin, a carotenoid present in animal and human diets, may have significant activity as an antioxidant against oxidative stress in tissues.