AbstractA subclone of an IgM‐producing hybridoma has been identified which has switched to producing an IgG1antibody. The parent hybridoma, PC‐140, produces an antibody which binds phosphorylcholine and reacts with monoclonal antibodies that recognize myelomas of the T‐15 idiotype. The IgG1antibody binds phosphorylcholine with the same affinity as the parental IgM and also reacts with the anti‐T‐15 monoclonal antibodies. While the IgM‐producing parent hybridoma does not express detectable surface IgM, the IgG1‐producing subclone produces both membrane and