Primary anti‐NP [(4‐hydroxy‐3‐nitrophenyl)acetyl] antibodies and NP‐specific receptors isolated from T cells of mice carrying the Ig‐1b allotype express the NPb idio‐type and a fine specificity marker (heteroclicity) which are both controlled by V genes in the heavy chain linkage group. While in antibodies both markers are invariably expressed in association with Ig light chains of the Λ type, antigen‐binding T cell‐derived receptors appear not to follow this rule in that they do not significantly bind to anti‐Λ antisera. Mice of the SJL strain also carry the Ig‐lb allotype but express a defect in Λ chain synthesis, and their primary anti‐NP response is thus almost completely devoid of Λ‐bearing heteroclitic molecules of the NPb idiotype. In contrast, both NPb idiotype and heteroclicity are expressed in the majority of T cell‐derived receptors. This result permits a positive discrimination between T cell‐derived receptors and humoral antibodies in the SJL strain and shows that V gene expression follows different rules in the T and the B cell compartment. The expression of Ig V regions in isolated T cell‐derived receptors was also analyzed with antisera binding to presumed “framework” determinants of the VH and the VΛdomains of Ig. A reaction of anti‐VH antiserum with isolated T cell receptors could clearly be demonstrated. The anti‐VΛ antiserum may bind to a minor fraction of the NP‐specific molecules in our T cell receptor preparations.