β₂-Microglobulin (β₂-m) is a highly conserved polypeptide (12,000 molecular weight; 12K) noncovalently associated with the heavy chain (45–48K) of the major histocompatibility complex (MHC) class I antigens. Its synthesis is required for expression of the HLA-A/B and H–2K/D heavy chains at the cell surface¹; β₂-m is also associated with the human cell-surface antigens T6 and M241 isolated from thymocytes^2–6. However, on the T leukaemic cell line MOLT-4 some of the T6 antigens contain a different 12K subunit, termed β_t (refs 3, 7, 8). Purified human β₂-m can exchange partially both with human β₂-m associated with HLA-antigens⁹, and with mouse β₂-m associated with murine alloantigens^{10, 11}. As MOLT-4 cells were grown in the presence of fetal calf serum (FCS) and as serum is known to contain some free β₂-m¹², we examined whether β_t was bovine β₂-m which had replaced endogenous β₂-m on the surface of the cell. Here we show both that β₂-m from FCS or human serum (HuS) used in cell culture can exchange with β₂-m on the cell surface, and that β_t is in fact bovine β₂-m.