A κ-light chain variable region (V(K) dominantly employed in the serum antibody response of A/J mice to streptococcal group A carbohydrate (GAC) has been termed VK1(GAC). Examination of in vitro recombinants between the isolated heavy chain and light chains of VK1(GAC+) and VK1(GAC)-anti-GAC hybridomas and non-GAC-binding myeloma proteins indicated that two antisera (anti-Id5 and anti-Id20) recognized the VK1(GAC) light chain when it was free in solution or paired with several heterologous heavy chains. Screening of a panel of A/J anti-GAC monoclonal antibodies with these antisera showed almost complete concordance between Id5 and Id20 expression and the presence of VK1(GAC) light chain as detected by its unique isoelectric focusing spectrotype. These antisera were used to examine serum expression of the VK1(GAC) light chain in normal and hyperimmune serum of A/J mice. Normal A/J serum contained from 20 to 100 μg Id5/ml serum, whereas only 1 to 10 μg Id20/ml serum was detected. The levels of both VK1(GAC) idiotypes increased dramatically 10- to 20-fold after hyperimmunization of mice with group A vaccine. When serum IgG from normal and immune mice was fractionated into the IgG subclasses (IgG1, IgG2a, and IgG3), it was found that the VK1(GAC) light chain does not pair randomly with heavy chains of the IgG subclasses, but rather is associated preferentially with heavy chains of the IgG3 subclass whether or not it is associated with antibodies to GAC. These results suggest that the heavy chain pairing exhibited by this V(K) product may not be random.