Abstract: The method for demonstrating the presence of an activity of an enzyme by(a) incubating said enzyme with a fluorogenic substrate A which is converted by the enzyme to a product B differing from A in respect to its fluorescent properties, A and/or B carrying a chromophore which is a triplet sensitizer having a triplet energy level above the excitation energy level of a lanthanide ion selected from the group consisting of Eu.sup.3+, Tb.sup.3+, Dy.sup.3+ and Sm.sup.3+ and which is capable of chelating said lanthanide ion by means of an oxygen or nitrogen atom in said chromophore, and that B differs from A either by(i) carrying a different chromophore, or(ii) having a different chelating ability, and(b) measuring the change in fluorescence caused by said enzyme.