Cells from mice immunized with SSS-III pneumococcal polysaccharide together with allogeneic lymphoid cells, a procedure which results in the transient appearance of cells which secrete both IgM and IgG3 anti-SSS-III antibodies were fused with myeloma line Sp 2/0. Hybridomas were isolated which secreted both IgM and IgG3 anti-SSS-III antibodies. These lines were "double producers" by the following criteria: (1) subclones were each double producers in vitro; (2) subclones transplanted into mice produced an IgM and IgG3 paraprotein each of which had antibody activity to SSS-III; (3) S-35 labeled antibodies of IgM and IgG3 types were produced by each subclone incubated in vitro with S-35 methionine; and (4) many individual cells stained with fluorochrome labeled antibodies to both igM and IgG3.