OBJECTIVE: B cell multiplication plays a key role in infections mononucleosis. The present study was designed to detect the expression of B-lymphocyte stimulator (BLyS) mRNA in peripheral blood using real-time fluorescence quantitative polymerase chain reaction (RFQ-PCR) in children with infectious mononucleosis in order to explore the role of BLys in this disorder. METHODS: Specific primers and TaqMan probes of BLyS were designed, and fluorescence of the PCR products were detected continuously during amplification. According to the standard curves created by plasmid DNA, the expression level of target genes in clinical samples were calculated using Stata Software version 8.0, and the results were presented as the ratio of copies of target gene mRNA to β2 microglobulin (β2M) mRNA copies. BLyS mRNA expression in peripheral blood was measured by RFQ-PCR in 18 children with infectious mononucleosis and the results were compared with those measured in 15 healthy controls. RESULTS: The range of target gene mRNA detected by REQ-PCR was from 109 ng/L to 101 ng/L. The coefficient of variation for intra-experimental and inter-experimental reproducibility ranged from 1.88% to 5.89% and 6.32% to 12.34%, respectively. BLyS mRNA expression in peripheral blood in children with infectious mononucleosis were significantly higher than that in controls (1.65±0.10 vs 0.56±0.08; P<0.01). CONCLUSIONS: RFQ-PCR has a high sensitivity and reproducibility for the measurement of BLyS mRNA expression. BLyS may be involved in the development of infectious mononucleosis.［Chin J Contemp Pediatr, 2007, 9 (6):553-556］