The effect of indoor airborne fungi on interleukin-4 (IL-4) mRNA expression in the peripheral blood mononuclear cells (PBMCs) of rats was studied by inoculating via trachea with fungi suspension. A simplified competitive RT-PCR method was used to measure the IL-4 mRNA expression in plasma. The results showed that interleukin-4 expression was much higher in the affected groups than that in the control one. It suggested that IL-4 might play an important role in the sensitization progress of fungi.