Antibodies 2S1.3 and 2S1.1 are hybridoma derived gamma 3K murine immunoglobulins with specificity for N-acetylglucosamine beta 1 leads to 3-linked to L-rhamnose, the immunodeterminant of the streptococcal group A polysaccharide. The VL 2S1.3 amino acid sequence is the second complete one reported with this specificity, and it is the first fully determined VK25 structure. The VK regions of this and the previously described monoclonal anti-A-CHO antibody 7S34.1 are compared with 17 selected partial and complete murine VK regions, preferentially from anti-polysaccharide antibodies. Both VK 2S1.3 and 7S34.1 increase the variability of existing murine VK regions; however, they are, as anti-A-CHO antibodies, most homologous to each other sharing the so far unique residue argine 74. The amino acid sequence of the constant region of 2S1.3 light chain was also determined. It was found to be identical with that of MOPC21, but different from MOPC41 and MOPC70 in residue positions 127, 129, 163 and 166.