Cloned rabbit T cell lines specific for the group-specific carbohydrate of group C streptococcus were established to study T cell responses to this antigen. T cell lines and clones of these lines were prepared from peripheral blood lymphoid cells of animals previously hyperimmunized with group C streptococcal vaccine. All cell lines studied were antigen specific in that they proliferated in the presence of group C but not group A vaccine. Three clones from one rabbit showed strong proliferation in the presence of soluble group C carbohydrate, in addition to the particulate vaccine. One of these clones, 5317.1, upon additional characterization, was shown to possess helper activity in that it induced autologous B cells to produce anti-carbohydrate antibody. Interestingly, B cells could not be stimulated by this carbohydrate-responsive clone to produce anti-carbohydrate antibody in the presence of soluble carbohydrate; the intact vaccine was required to demonstrate the helper effect of the clone. Phenotypic characterization of this clone with a panel of monoclonal antibodies indicated that it was of the T1 subpopulation of peripheral T cells. No suppressor activity was observed for any of the lines studied. Although the use of outbred animals in T cell cloning experiments has certain disadvantages, the present results indicate that cloned rabbit T cells may be useful tools for the elucidation of phenotypic and functional differences of rabbit T cell subsets.