The M54 transgenic mouse line, which carries the 17.2.25 Ig mu heavy chain gene, rearranges Ig heavy chains and expresses both transgenic and endogenous mu. B cell lineage development is selectively impaired in these mice and cells that simultaneously express transgenic and endogenous mu ('double-producers') are common amongst the B cells and plasma cells that do develop. Weaver, Imanishi-kari, Baltimore and colleagues failed to obtain double-producing hybridomas from M54 mice; however, molecular and serologic studies presented here show that such hybridomas are readily generated. These hybridomas are extremely unstable and rapidly yield variants producing either transgenic or endogenous mu. Therefore the stable cloned lines we obtained, like Weaver et al., were almost all single or non-producers. We also found that the VH gene usage in our hybridomas was skewed towards the JH proximal (VHQ52, VH81X) families, supporting the idea that the expression of the M54 transgene alters the endogenous Ig repertoire.