The technique of polyethylene glycol mediated cell fusion was used to establish 22 monoclonal cell lines secreting anti-(T,G)-A--L antibody. Cell lines were derived from C3H.SW and B10 mice and produced antibody with kappa light chains and predominantly gamma 1, heavy chains. Fine-specificity analysis demonstrated that 15 cell lines made antibodies that also recognize a determinant present on GAT, GT (9:1) and GT (1:1), whereas little, if any, serum antibody demonstrates this cross-reaction. Fourteen antibodies, derived from both B10 and C3H.SW mice, bear idiotypic determinants defined by Lewis anti-[B10 anti-(T,G)-A--L], but only two, both from C3H.SW mice, react with Lewis anti-[C3H.SW anti-(T,G)-A--L]. Adsorption studies indicate that no hybridoma tested bore the complete set of idiotypic determinants defined by either serum.