A calorimetric method was used to monitor the relative promoter activity of the chromosome DNA fragments cloned from Pseudomonas maltophilia AT18 in Escherichia coli. The promoter probe vector, plasmid pKK232-8, was used to form the recombinants. Three recombinants were selected to study by calorimetric method and agar plate method. The relative strength of the promoter was represented by the resistance level to chloramphenicol. The results showed that the agar plate method could only provide the MIC of the antibiotic to the recombinants, but the calorimetric method not only provided a more precise MIC but also IC50 that quantitates the activity of these promoter fragments. (C) 2005 Elsevier B.V. All rights reserved.