method of the present invention the change in the activity upon binding to streptavidin was compared between biotinated firefly luciferase bl203 and the chemically modified biotinated firefly luciferase 0 1 ng ml of each biotinated firefly luciferase in a luciferase diluent 1 bsa 1 mm edta 1 mm u03b2 mercaptoethanol 50 mm hepes ph 7 5 was mixed with an equal volume of 1 u03bcg ml streptavidin boehringer mannheim gmbh in the luciferase diluent or with the luciferase diluent as the control and allowed to stand for 30 min at room temperature 50 u03bcl of each sample solution was put to a well on microtiter plate microlite 2 produced by dynatech laboratories which was then attached to a microplate reader luminous ct 9000d dia iatron for bioluminescent and chemiluminescent measurements and 50 u03bcl substrate solution 40 mm atp 1 4 mm luciferin 300 mm magnesium sulfate 50 mm hepes ph 7 5 was put to each well and the number of photons generated for 10 seconds was determined the result showed that after mixed with streptavidin the remaining activities of biotinated firefly luciferase bl203 and the chemically modified biotinated firefly luciferase were 93 and 62 respectively indicating that the change of the activity was small in the case of the biotinated firefly luciferase bl203 of the present invention while nearly 40 of the activity was lost in the case of the chemically modified biotinated firefly luciferase 100 u03bcl of 5 u03bcg ml goat anti mouse igg fc fragment specific polyclonal antibody produced by.