Abstract: The invention is directed to monoclonal antibodies reactive with a member of the V.beta.3 family variable region of the beta chain of the TCR. More particularly, the invention provides for detection of the V.beta.3.1 subfamily. In a specific embodiment the invention provides for detection of V.beta.3.1, without cross-reacting with other V.beta.3 family variable regions. In a specific embodiment, the monoclonal antibodies of the invention do not react with V.beta.3.2. In particular, the invention provides monoclonal antibodies, termed 5E4 and 8F10, which react with the variable region of a member of the V.beta.3 family. In various embodiments of the invention, these antibodies, or fragments or derivatives thereof, can be used to bind with a member of the V.beta.3 family TCR variable region amino acid sequences, either as part of an intact TCR or peptide, or T cell-surface molecule, or a fragment thereof.