The quorum sensing (QS) mechanism of Pseudomonas aeruginosa has been studied extensively due to its involvement in cystic fibrosis, a deadly disease that is responsible for the death of more than a thousand people annually. In order to develop biochemical assay method for screening QS inhibitor, we have studied the production and characterization of recombinant LasR protein, which is a transcriptional activator for the QS mechanism in P. aeruginosa. In recombinant Escherichia coli BL21, LasR was produced as functionally-active proteins when the cells were cultivated in the presence of a proper signaling molecule (acyl homoserine lactone, AHL) only. Some soluble LasR proteins could be obtained from the cells which were grown in AHL-deficient medium, but they did not show binding affinity to the promoter sequence OP1 (lasB elastase promoter). Furthermore, the active LasR, presumably produced as LasR-AHL complex, was not dissociated into its components (LasR and AHLs) in vitro. The current results indicate that the production of pure and active LasR devoid of AHL is very difficult. It can be concluded that the development of biochemical assay method for screening AHL competitive inhibitors which requires pure and active LasR proteins might not be possible unless the structure of LasR and/or its folding processes is modified.