The mixed lymphocyte reaction (MLR) is the most commonly used method of investigating the immunological response of lymphocytes. However, this conventional technique requires the use of a radioisotope and is time consuming. Furthermore, there is a possibility that MLR does not reflect the response of each lymphocyte subset individually. The purpose of this study is to determine whether detection of IL-2 mRNA by reverse transcription polymerase chain reaction (RT-PCR) can be used as a new method of studying a precise response of PEL to allo-antigens. The study was conducted using lymphocytes from healthy volunteers, a renal transplant recipient who exhibited donor specific MLR unresponsiveness, the donor and the recipient's father. A kinetics study revealed that the expression of IL-2 mRNA in the MLC reached its maximum at 24-48 h. Lymphocytes from a renal transplant recipient also exhibited the same IL-2 mRNA expression pattern in response to stimulation with PEL from a non-related 3rd party but not to that from the donor. These data suggest that the expression level of IL-2 mRNA in MLC measured by RT-PCR at 24-48 h provides accurate response of PEL to allo-antigens.