The relationship of chemical structures of 6-formylpterin (6FP) and its derivatives with scavenging activity of singlet oxygen (¹O₂) was examined. First, effects of pterin derivatives on ¹O₂ released from activated human neutrophils were examined. The neutrophils, stimulated with opsonized zymosan, released ¹O₂ that was detected by chemiluminescence using a ¹O₂ specific probe, trans -1-(2′-methoxyvinyl)pyrene. 6FP and its derivatives suppressed the ¹O₂ release. 6FP and other commercially available pterin derivatives, such as biopterin and neopterin, which have different substitutions at the 6-position, suppressed the ¹O₂ release with similar extent. On the other hand, newly synthesized pterin derivatives, which have different substitutions at the 2- and/or 3-position, such as 2-amino-6-formyl-3-methylpteridin-4-one, suppressed the ¹O₂ release in a dose-dependent manner and more potently than 6FP. Then, the ¹O₂ scavenging activity of pterin derivatives was examined photochemically by direct analysis of near-infrared luminescence at 1270 nm, the most sensitive method for the detection of ¹O₂. When rose Bengal, a photosensitizer, in D₂O solution, was irradiated by 514 nm laser beam, the emission spectrum of ¹O₂ was observed. 6FP suppressed this emission spectrum of ¹O₂, and the newly synthesized pterin derivatives with different substituent at the 2- and/or 3-position suppressed the spectrum more potently than 6FP. The order of potency was similar to that obtained from biological assays. These findings indicate that the substitutions at the 2- and/or 3-position play an important role in ¹O₂ scavenging activity of pterin derivatives.