The present invention provides a novel polypeptide having a .beta.1,3-N-acetylglucosaminyltransferase activity, an agent for synthesizing a sugar chain comprising the polypeptide, a process for producing a sugar chain or a complex carbohydrate using the agent for synthesizing a sugar chain, DNA encoding the polypeptide, a process for producing the polypeptide, an antibody against the polypeptide, and a diagnosis method and a medicament for treatment for inflammation, cancer or tumor metastasis using the DNA or the antibody. The present invention is useful for synthesis of a useful sugar chain and diagnosis and treatment for inflammatory diseases, cancer or tumor metastasis.