Carbon monoxide (CO) is endogenously produced by haeme oxygenase-1 and has profound effects on intracellular signalling processes, generating anti-inflammatory, antiproliferative and antiapoptotic effects. A boron-containing compound CORM-A1 is capable of releasing CO in such a way to mimic physiological functions of haeme oxygenase-1. Considering the importance of Th1/Th17 versus Th2 balance in the final outcome of immune and inflammatory responses in this study we focused on immune-modulatory effects of CORM-A1 on murine lymph node–derived T cells in vitro and its influence on T-cell proliferation, activation and differentiation. Anti-CD3/CD28 antibody-triggered lymph node cells proliferation remained unaffected after 24-hour CORM-A1 treatment, as well as the expression of the early activation marker CD25. However, CORM-A1 successfully reduced the secretion of the two representative pro-inflammatory cytokines, IFN-γ and IL-17, while the secretion of anti-inflammatory cytokine IL-4 remained unchanged. Furthermore, CORM-A1 efficiently reduced the percentage of CD4+IFN-γ+ and CD4+IL-17+ cells, whereas CD4+IL-4+ cell population increased after treatment. Also, CORM-A1 significantly reduced expression of transcription factor RORγT, necessary for Th17 development, but the expression of Th1-related and Th2-related transcription factors (T-bet and GATA-3, respectively) remained unchanged. In conclusion, our findings indicate that CO has anti-inflammatory role through the regulation of balance between pro-inflammatory Th1/Th17 and anti-inflammatory Th2 cells. Observed immunomodulatory effects of CORM-A1 could be useful for developing novel therapeutic approaches in managing Th1/Th17-mediated immune disorders.