The complete nucleotide sequences of the cDNAs encoding the heavy- and light-chain variable regions (VH and VL) of myeloma protein W3129, an anti-alpha(1----6)-dextran with a unique combining site, have been determined. The VH region is encoded by a germ-line gene highly homologous to VH441, which also appears to be used in some anti-galactans and anti-levans. W3129 VH uses a diversity region that is not found in any of the galactan- or levan-specific antibodies and whose germ-line counterpart is unknown. The W3129 light chain differs from complete light-chain sequences thus far reported but is identical in the first 23 amino acids with NZB IgA myeloma PC118.