The T-cell antigen receptor consists of a disulfide-linked heterodimer (Ti) that is associated with another set of three nonpolymorphic, noncovalently linked peptides termed "T3." The cell surface expression of T3 has been thought to depend upon association with Ti. In this study, we demonstrate that T3 can be expressed in the absence of an associated Ti molecule on a T-cell leukemic line, PEER. Instead, on this cell line, T3 appears to be expressed in association with a 55- to 60-kDa glycoprotein that has a peptide backbone of 29 kDa. PEER fails to express Ti alpha-chain transcripts but does express Ti beta- and gamma-chain transcripts. Using a monoclonal antibody that reacts with nonpolymorphic epitopes expressed on Ti, WT31, we demonstrate that PEER fails to react with this antibody but does react with three independently derived anti-T3 antibodies. Moreover, a small subpopulation of T3-positive peripheral blood lymphocytes, like PEER, fails to express the antigenic determinants recognized by WT31. These results suggest that, on these normal lymphocytes, T3 may likewise be associated with a non-Ti molecule. The possibility that the 55- to 60-kDa molecule expressed on PEER, termed "Tp55-60," represents the protein product of the previously identified Ti gamma-chain gene is discussed.