Protein arginine N-methyltransferases (PRMTs) act in signaling pathways and gene expression by methylating arginine residues within target proteins. PRMT1 is responsible for most cellular arginine methylation activity and can work independently or in collaboration with other PRMTs. In this study, we demonstrate a direct interaction between PRMT1 and PRMT2 using co-immunoprecipitation, bimolecular fluorescence complementation, and enzymatic assays. As a result of this interaction, PRMT2 stimulated PRMT1 activity, affecting its apparent V(max) and K(M) values in vitro and increasing the production of methylarginines in cells. Active site mutations and regional deletions from PRMT1 and -2 were also investigated, which demonstrated that complex formation required full-length, active PRMT1. Although the inhibition of methylation by adenosine dialdehyde prevented the interaction between PRMT1 and -2, it did not prevent the interaction between PRMT1 and a truncation mutant of PRMT2 lacking its Src homology 3 (SH3) domain. This result suggests that the SH3 domain may mediate an interaction between PRMT1 and -2 in a methylation-dependent fashion. On the basis of our findings, we propose that PRMT1 serves as the major methyltransferase in cells by forming higher-order oligomers with itself, PRMT2, and possibly other PRMTs.