The human T-cell leukaemia and differentiation antigen HTA 1 is defined by the monoclonal antibody NA1/34 (ref. 1) and also recognized by the monoclonal antibody OKT62. Like class I products of the human major histocompatibility complex, it has a glycosylated heavy (α) chain of approximately 45-50,000 molecular weight (MW) in non-covalent association with β2-microglobulin (β2m) (MW 11,900). A particular feature of HTA 1 is the presence in significant amounts of an additional β2m-like subunit, called βt (refs 3,4). To facilitate biochemical studies we have prepared a high HTA 1 expressor variant (NH17) of the human thymoma line MOLT-45. The N-terminal amino acid sequence of the βt purified from this cell line was shown to be indistinguishable from that of bovine β2m. Further, βt was present when the cells were grown in medium containing fetal calf serum (FCS), but absent from cells grown with human serum (HuS). We show here that addition of human and bovine β2m to MOLT-4 and NH17 cells grown in serum-free medium produces a significant elevation of HTA 1 antigen expression, providing evidence for a regulatory or stabilizing function for the exchange of extracellular β2m with a cell-surface antigen.