MOPC 104E is a mouse plasmacytoma which produces an IgM (lambda) myeloma protein. This IgM protein has a binding specificity for dextran B-135s. This report describes a variant of the MOPC 104E tumor which secretes a myeloma protein which does not react with the dextran. The apparent variant of MOPC 104E arose spontaneously during routine passage of the MOPC 104E tumors in Balb/C mice and appears to be stable.
