The transcription factor AP-1 plays an important role in cellular proliferation, transformation and death. In this study, we report a novel human gene, AC3-33 (GenBank name: c3orf33, FLJ31139), which encodes a secretory protein that can inhibit Elk1 transcriptional activity via ERK1/2 pathway. The AC3-33 mRNA encodes a protein of 251 amino acids, which is a classical secretory protein. Functional investigation reveals that overexpression of AC3-33 significantly inhibit AP-1 activity and DNA-binding ability. Further investigation indicated that overexpression of AC3-33 significantly inhibit transcriptional activity of Elk1 and c-jun, but not c-fos. As for the upstream of signaling pathway of Elk-1, our study demonstrated that overexpression of AC3-33 significantly down-regulates phosphorylation of ERK1/2, but not JNK/SAPK or p38 MAPK. These results clearly indicate that AC3-33 is a novel member of the secretory family and inhibits Elk1 transcriptional activity via ERK1/2 MAPK.