We recently reported that kinobeon A, produced from safflower cells, suppressed the free radical-induced damage of cell and microsomal membranes. In the present study, we investigated whether kinobeon A quenches singlet oxygen, another important active oxygen species. Kinobeon A inhibited the singlet oxygen-induced oxidation of squalene. The second-order rate constant between singlet oxygen and kinobeon A was 1.15 x 10(10) M(-1)s(-1) in methanol containing 10% dimethyl sulfoxide at 37 degrees C. Those of alpha-tocopherol and beta-carotene, which are known potent singlet oxygen quenchers, were 4.45 x 10(8) M(-1)s(-1) and 1.26 x 10(10) M(-1)s(-1), respectively. When kinobeon A was incubated with a thermolytic singlet oxygen generator, its concentration decreased. However, this change was extremely small compared to the amount of singlet oxygen formed and the inhibitory effect of kinobeon A on squalene oxidation by singlet oxygen. In conclusion, kinobeon A was a strong singlet oxygen quencher. It reacted chemically with singlet oxygen, but it was physical quenching that was mainly responsible for the elimination of singlet oxygen by kinobeon A. Kinobeon A is expected to have a preventive effect on singlet oxygen-related diseases of the skin or eyes.