Sterigmatocystin (ST) is a toxic metabolite mainly produced by the fungi Aspergillus nidulans and Aspergillus versicolor. ST is considered a potent carcinogen, mutagen and teratogen. However, over the past few years, it has been demonstrated that it is less acutely toxic to rodents in vivo. In this study, we evaluated the putative effects of ST on the expression of tumor necrosis factor-α (TNF-α), interleukin (IL)-6 and IL-12 at mRNA levels in murine peripheral blood mononuclear cells (mPBMCs) and peritoneal macrophage cells and on the serum TNF-α and IL-6 levels in BALB/c mice. Our results show the downregulation of TNF-α, IL-6 and IL-12 mRNA expression in mPBMCs and peritoneal macrophage cells using semi-quantitative reverse transcription-polymerase chain reaction following ST treatment by intraperitoneal injection. Additionally, serum TNF-α and IL-6 levels were also decreased as shown by enzyme-linked immunosorbent assay (ELISA). These results suggest that ST contamination has negative immunomodulatory effects through the downregulation of cytokine expression and secretion.