We found that JBP1, known as a human homolog (Skb1Hs) of Skb1 of fission yeast, interacts with NS3 of the hepatitis C virus in a yeast two-hybrid screen. Amino acid sequence analysis revealed that Skb1Hs/JBP1 contains conserved motifs of S-adenosyl-l-methionine-dependent protein-arginine methyltransferases (PRMTs). Here, we demonstrate that Skb1Hs/JBP1, named PRMT5, is a distinct member of the PRMT family. Recombinant PRMT5 protein purified from human cells methylated myelin basic protein, histone, and the amino terminus of fibrillarin fused to glutathione S-transferase. Myelin basic protein methylated by PRMT5 contained monomethylated and dimethylated arginine residues. Recombinant glutathione S-transferase-PRMT5 protein expressed in Escherichia coli also contained the catalytic activity. Sedimentation analysis of purified PRMT5 on a sucrose density gradient indicated that PRMT5 formed distinct homo-oligomeric complexes, including a dimer and tetramer, that comigrated with the enzyme activity. The PRMT5 homo-oligomers were dissociated into a monomer in the presence of a reducing agent, whereas a monomer, dimer, and multimer were detected in the absence or at low concentrations of a reducing agent. The results indicate that both covalent linkage by a disulfide bond and noncovalent association are involved in the formation of PRMT5 homo-oligomers. Western blot analysis of sedimentation fractions suggests that endogenous PRMT5 is present as a homo-oligomer in a 293T cell extract. PRMT5 appears to have lower specific enzyme activity than PRMT1. Although PRMT1 is known to be mainly located in the nucleus, human PRMT5 is predominantly localized in the cytoplasm.