IgM can be obtained in the form of a pentamer by placing the genes encoding the H, L, and J chains on the same vector to transform appropriate host cells. The gene encoding the J chain may be introduced by co-transfection. When no J chain is expressed, the IgM is produced as a hexamer. The transformants obtained according to the present invention achieve a high yield of IgM. The present invention also provides methods which enable separation and quantification of polymeric IgM.