Abstract: An active oxygen scavenger comprising as an active ingredient a pterin derivative of the formula (I): ##STR1## wherein R.sup.1 and R.sup.2 are independently a hydrogen atom, an alkyl group of 1 to 4 carbon atoms, or an acyl group of the formula R.sup.3 --CO--, R.sup.3 is an alkyl group of 1 to 4 carbon atoms, X is a formyl group or hydroxymethyl group, A is a group of the formula (Ia): ##STR2## and n is 0 or an integer of 1 or more, with the proviso that when X is a hydroxymethyl group, n is 0, or when n is an integer of 1 or more, each of R.sup.1 and R.sup.2 is a hydrogen atoms, and X is a formyl group, or a cyclic compound thereof, or a salt thereof is disclosed.