Singlet molecular oxygen was generated by illumination of phenosafranin in phosphate buffer at pH 7.5. Relative efficiencies of various imidazole compounds to form endoperoxides were assayed by following at 25 degrees C the rate of light- and imidazole-dependent bleaching of N,N-dimethyl-4-nitrosoaniline. Of over 30 imidazole compounds tested, imidazole-4-acetic acid, a major catabolite of histamine in mammals, exhibited the highest activity. L-Carnosine (beta-alanyl-L-histidine), a natural dipeptide prevalent in striated muscle of mammals, possessed several properties important for a physiologically significant scavenger of singlet oxygen. On a molar basis, this readily water-soluble C-terminal histidine dipeptide reacted with singlet oxygen two- to four-fold faster than free L-histidine and approximately two-fold faster than the N-terminal L-histidine dipeptides tested. Furthermore scavenging ability of L-carnosine did not appreciably increase or decrease with time of reaction, in contrast to behaviors exhibited by a number of other imidazole compounds that included some other C-terminal L-histidine dipeptides. The fungal metabolite, ergothioneine, blocked singlet oxygen generation by illuminated phenosafranin.