A series of 6 hybrid clones are described which produced antibodies directed to murine IgM. These were derived by immunizing rats with W279 B-lymphoma cells and then fusing the immune spleen cells to enzyme-deficient murine plasmacytoma cell lines. Immunoenzymatic techniques were used to select clones producing desirable antibodies to cell surface or soluble antigens. Antibodies from 3 of the clones (293.14, 414.18 and 307.5) preferentially bound to idiotypic determinants of W279 immunoglobulin whereas the remaining 3 clones made antibodies which bound to various myeloma and normal cell surface IgM molecules. One IgM-specific clone (287.2) produced antibody of relatively low avidity. The two other clones (81.20 and 331.12) which secreted anti-mu antibodies appeared to be directed to determinants which differed in glutaraldehyde lability and expression in F(ab')2 preparations.