Antibodies to framework determinants of the VH and Vλ. fragments of MOPC 315 and antisera to the VH idiotype determinants of the A 5 A antibody were used to analyze the antigen receptors of mouse T (and B) cells. This was done by using the antibodies as inhibitors in (a) an assay in which the binding of radiolabeled streptococcal carbohydrate (A‐CHO) antigen by primed and unprimed T and B cells is determined and (b) an assay in which the helper activity of group A streptococcal vaccine‐primed T cells is determined. The results suggest that the major proportion of primed and unprimed T cells binding A‐CHO (70–9076) exhibit VH framework and VH idiotypic determinants. This population appears to include the helper T cells. A minor proportion of T cells (10–3076) express Vλ‐related framework determinants and lack VHframework and VH idiotypic determinants. This population does not include T helper cells. Taken together, the data suggest that a subpopulation of T cells, including the helper cells, uses entire Ig VH regions as part of their antigen receptor system.