The objective of this study was to investigate the expression of surface markers on T lymphocytes and the Th1/Th2 immune response in pulpal inflammation associated with specific bacteria. Pulpal inflammation was experimentally induced in rat mandibular incisors by drilling, without coolant, to open pulp chambers. Streptococcus mutans (S. mutans group), Porphyromonas endodontalis (P. endodontalis group), and a sterile cotton pellet only (control group) were inoculated in the canal. The expression of CD25 and CD54 on CD4+, and CD8+ lymphocytes in pulp tissues was determined by using a flow cytometer. The levels of interleukin (IL)-2, interferon (IFN)-gamma, and IL-4 were measured by ELISA. Flow-cytometric analysis showed that the mean ratio of CD4+:CD8+ was 0.96 in the control group, 0.99 in the S. mutans group, and 0.52 in the P. endodontalis group. An increase in CD25 and CD54 expression on CD4+ T lymphocytes was related to the bacterial infection (p < 0.05) and accompanied an increase in IL-2 concentration. The higher concentration of IFN-gamma than IL-4 in the P. endodontalis group suggested a Th1 reaction in the early stage of pulpal inflammation induced by P. endodontalis.