FeCl2 was found to be a useful catalyst for the determination of H2O2 based on the chemiluminescence of a Cypridina luciferin analogue, 2-methyl-6-phenyl-(p-methoxyphenyl)-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (MCLA). By this method, H2O2 was detected in a range from 300 pmol/ml to 10 nmol/ml. In addition, glucose oxidase (GOD) was determined in a range from 1.18 x 10(-5) units/ml (280 amol/ml) to 7.56 x 10(-4) units/ml (17.9 fmol/ml) when GOD was incubated with glucose for 10 min at 34-degrees-C.