Idiotypes of inulin-binding myeloma proteins (InuBMP) were determined primarly by variable region light chains (VL) or by variable region heavy chains (VH) but needed both chains to be expressed. Recombinant molecules were used to show that individual idiotypes (IdI) of U61, E109, T957, and A4 InuBMP and cross-specific idiotypes (IdXB) of U61 were primarily determined by VL while cross-specific idiotype (IdXA) of A4 was determined mainly by VH. The assignment of genes controlling idiotypes to VH based on allotype linkage (e.g., IdXB) is dubious until the role of the L chain in determining that idiotype is assessed. IdXB has been shown to be a VL-VH marker which presumably is controlled by two unlinked genes. However IdXB can be used as a L chain marker in combinations of strains differing in their L chain genes but having the same permissive H chain genes. Conversely IdXB can be used as a H chain marker in strains having the same permissive L chain genes but differing in their H chain genes.