We show here that C1q suppresses IL‐12p40 production in LPS‐stimulated murine bone marrow‐derived dendritic cells (BMDC). Serum IL‐12p40 concentration of C1q‐deficient mice was higher than that of wild‐type mice after intraperitoneal LPS‐injection. Because neither globular head of C1q (gC1q) nor collagen‐like region of C1q (cC1q) failed to suppress LPS‐induced IL‐12p40 production, both gC1q and cC1q, and/or some specialized conformation of native C1q may be required for the inhibition. While C1q did not affect mRNA expression of Toll‐like receptor 4 (TLR4), MD‐2, and myeloid differentiation factor 88 (MyD88), BMDC treated with C1q showed the reduced activity of NF‐κB and the delayed phosphorylation of p38, c‐Jun N‐terminal kinase, and extracellular signal‐regulated kinase after LPS‐stimulation. CpG oligodeoxynucleotide‐induced IL‐12p40 and TNF‐α production, another MyD88‐dependent TLR‐mediated signal, was also suppressed by C1q treatment. Therefore, C1q is likely to suppress MyD88‐dependent pathway in TLR‐mediated signals. In contrast, C1q failed to suppress colony formation of B cells responding to LPS or LPS‐induced CD40 and CD86 expression on BMDC in MyD88‐deficient mice, indicating that inhibitory effects of C1q on MyD88‐independent pathways may be limited. Taken together, C1q may regulate innate and adaptive immune systems via modification of signalsmediated by interactions between invading pathogens and TLR.