(EN) An enzyme assay reagent comprises a substance which is capable of interaction with the enzyme to give rise to the formation of a compound of formula X-A-Y-NO2, wherein A comprises an aromatic nucleus, X comprises an auxochromic group and Y comprises a unsaturated group which is capable of transmitting electron resonance between the aromatic nucleus and the nitro substituent, said compound per se being capable of producing a visual signal which is easily discernible by eye. The reagent may be used in a through-flow device comprising an upper incubation compartment (1) and a lower visualisation compartment (2) held together by pivot pin (11); the lower compartment contains a quantity of fibrous cellulose (3) sandwiched between upper (4) and lower (5) layers of cotton wool. (FR) Un reactif d"essai d"enzymes comprend une substance capable de reagir avec l"enzyme pour donner lieu a la formation d"un compose de formule X-A-Y-NO2, ou A comprend un noyau aromatique, X comprend un groupe auxochromique et Y comprend un groupe insature capable de transmettre la resonance d"electrons entre le noyau aromatique et le substituant nitro, ledit compose per se etant capable de produire un signal visuel facilement perceptible a l"oeil.