A novel bioassay system based on RT-PCR has been established to search for biologically active compounds which can modulate the expression of genes encoding important regulatory proteins. Changes in target mRNA level in the treated cell cultures were quantitatively determined by competitive PCR using internal standard DNA. The compounds discovered using this bioassay will be useful tools to elucidate the mechanisms of cellular signaling guiding the expression of the target genes. As the first application of the new bioassay strategy, the expression of the interleukin-2 gene in Jurkat cells, a human T cell line, was investigated. Screening of several crude drugs used in traditional Chinese medicines demonstrated that the aqueous acetone extract of Coptidis Rhizoma enhanced the expression of the interleukin-2 gene by about 5-fold as compared to the control.