Lycopene, a biologically occurring carotenoid, exhibits the highest physical quenching rate constant with singlet oxygen (kq = 31 X 10(9) M-1 s-1), and its plasma level is slightly higher than that of beta-carotene (kq = 14 X 10(9) M-1 s-1). This is of considerable general interest, since nutritional carotenoids, particularly beta-carotene, and other antioxidants such as alpha-tocopherol (kq = 0.3 X 10(9) M-1 s-1) have been implicated in the defense against prooxidant states; epidemiological evidence reveals that such compounds exert a protective action against certain types of cancer. Also, albumin-bound bilirubin is a known singlet oxygen quencher (kq = 3.2 X 10(9) M-1 s-1). Interestingly, those compounds with low kq values occur at higher plasma levels. When these differences are taken into account, the singlet oxygen quenching capacities of lycopene (0.7 microM in plasma), beta-carotene (0.5 microM in plasma), albumin-bound bilirubin (15 microM in plasma), and alpha-tocopherol (22 microM in plasma) are of comparable magnitude.