2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is the predominant heterocyclic amine formed in cooked meat and fish and causes cancers in the colon, the mammary glands, and the lymphoid organs. In the present study, we investigated the immunological impact of PhIP using thymocytes isolated from Balb/c mice and a murine thymocyte-derived cell line, EL4. Treatment of the thymocytes with PhIP moderately inhibited T-cell mitogen-induced cell proliferation and interleukin (IL)-2 secretion. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that PhIP attenuated IL-2 mRNA expression in the thymocytes and EL4 cells stimulated with phytohemagglutinin (PHA) plus phorbol 12-myristate 13-acetate (PMA). In vitro transient transfection assay using a reporter gene construct containing IL-2 promoter showed that the decrease in the steady-state IL-2 mRNA level by PhIP is partially due to the attenuation of IL-2 mRNA synthesis at the transcriptional level. Furthermore, an electrophoretic mobility shift assay showed that PhIP inhibited DNA binding activity of nuclear factor for immunoglobulin kappa chain in B cells (NF-kappaB), activator protein-1 (AP-1) and nuclear factor of activated T cells (NF-AT), which are known to be responsible for IL-2 transcriptional activation. Concomitantly, PhIP inhibited the PMA/PHA-induced generation of reactive oxygen species (ROS) involved in activation of the transcription factors. These results suggest that PhIP has potential immunosuppressive effects by inhibiting T-cell proliferation and IL-2 expression through down regulation of ROS generation and thereby inhibiting NF-kappaB, AP-1 and NF-AT activation.