Antibodies directed against the β chain of the T‐cell receptor (TCR) have been detected in animals and in humans in a number of distinct immune states that do not involve direct immunization with either T cells or TCR epitopes. When C57Bl/6 mice are infected experimentally with the LP‐BM5 retrovirus mixture they produce increased titres of autoantibodies directed against TCR Vβ complementarity determining region 1 (CDR1) epitopes. Here, the authors utilized hybridoma technology to isolate monoclonal immunoglobulin (Ig)M antibodies (MoAbs) that arose at the peak of infection. The authors characterized the binding specificity tested using synthetic peptides modelling the CDR1 segments of 24 distinct Vβ gene products and determined the V_H gene usage by two such monoclonals. One binds to a restricted set of TCR Vβ CDR1 peptides, and the second reacts with approximately half of the CDR1 peptide homologues. These MoAbs are specific for T‐cell receptor β chains and do not bind to immunoglobulin light chains or to unrelated protein molecules. Both MoAbs bind to intact T cells expressing the Vβ domain (human Vβ8 and mouse Vβ11) from which selection peptides were derived, and costimulate a Vβ specific in vitro T cell proliferative response induced by the staphylcoccal enterotoxin E (SEE) superantigen.