The applicability of β- glucuronidase and chloramphenicol acetyltransferase reporter genes to a carnation (Dianthus caryophyllus L.) transformation procedure, was analyzed. Transgenic tobacco (Nicotiana tabacum L.) plants expressing the respective reporter genes were prepared and used as the enzyme source. Carnation leaf extract strongly inhibited enzymatic activity of β- glucuronidase, but not that of chloramphenicol acetyltransferase. One or more carnation phenolic compounds, acting in a noncompetitive manner, is suggested as the cause of the observed inhibition of fluorometrically assayed β- glucuronidase activity. This inhibition was eliminated by treating the carnation leaf extract with polyvinylpolypyrrolidone.