Intestinal epithelial cells are an important source of many biologically active molecules that modulate immune responses in the mucosa. The purpose of this study was to demonstrate the synthesis of complement C3 component in the rat non-transformed crypt-like intestinal epithelial cell line, IEC-6. Unstimulated IEC-6 cells secreted a low level of C3 protein and showed weak expression of C3 mRNA. The addition of interleukin (IL)-1 beta induced a dose- and time-dependent increase in C3 production. These effects of IL-1 beta were observed at a concentration as low as 0.01 ng/ml and reached a plateau at a concentration of 5 ng/ml. The effects were observed at the mRNA level as early as 6 h after the beginning of incubation. Transforming growth factor (TGF)-beta alone had no effect. However, TGF-beta at low concentrations (0.001-1 ng/ml) enhanced the effect of IL-1 beta in increasing C3 production; this enhancement was not observed at high concentrations (5-10 ng/ml). These effects of TGF-beta were also observed at the mRNA level. The present findings indicate that intestinal epithelial cells are indeed capable of synthesizing complement C3 in response to IL-1 beta and TGF-beta.